SUMMARY

        This summary does not contain all the information that you should consider before investing in our common stock. You should read this entire prospectus supplement and the accompanying prospectus, including the documents incorporated by reference herein and therein, before making an investment decision.

Our Company

        Our primary business is the production and sale of electric energy, capacity and ancillary services. We operate a portfolio of generation assets that is diversified in terms of dispatch profile, fuel type and geography. Our Coal and IPH segments are fleets of baseload coal facilities located in Illinois that dispatch around the clock throughout the year. Our Gas segment operates both intermediate and peaking natural gas facilities, located in the Midwest, the Northeast and California. In addition to generating power, our generating facilities also receive capacity revenues through structured markets or bilateral tolling agreements, as local utilities and ISOs seek to ensure sufficient generation capacity is available to meet future market demands.

        We sell electric energy, capacity and ancillary services on a wholesale basis from our power generation facilities. In connection with the acquisition of AER and its subsidiaries on December 2, 2013, we began serving residential, municipal, commercial and industrial consumers through our Homefield Energy retail business in Illinois. Wholesale electricity customers will, for reliability reasons and to meet regulatory requirements, contract for rights to capacity from generating units. Ancillary services are the products of a power generation facility that support the transmission grid operation, follow real-time changes in load and provide emergency reserves for major changes to the balance of generation and load. Retail electricity customers purchase energy and these related services in the deregulated retail energy market. We sell these products individually or in combination to our customers for various lengths of time ranging from hourly to multi-year transactions.

        We do business with a wide range of customers, including: regional transmission organizations and ISOs, integrated utilities, municipalities, electric cooperatives, transmission and distribution utilities, power marketers, financial participants such as banks and hedge funds, and residential, commercial and industrial end-users. Some of our customers, such as municipalities or integrated utilities, purchase our products for resale in order to serve their retail, commercial and industrial customers. Other customers, such as some power marketers, may buy from us to serve their own wholesale or retail customers or as a hedge against power sales they have made.

Recent Developments

The Acquisitions

        In August 2014, we entered into an agreement with Duke to purchase certain of its facilities located in the Midwest and its retail energy business and we entered into an agreement with ECP to purchase ERC and Brayton. The Acquisitions will increase our generation capacity by approximately 12,500 MW to nearly 26,000 MW. We anticipate that the Duke Midwest Acquisition and ERC and Brayton Acquisitions will each close by the end of the first quarter of 2015. However, we cannot assure you that the Acquisitions will be consummated or, if consummated, that they will be consummated at the price, within the time period or on the terms and with the anticipated benefits contemplated by this prospectus supplement. The closing of one Acquisition is not contingent upon the closing of the other Acquisition and this offering is not contingent on either or both of the Acquisitions occurring on a timely basis or at all. See "Risk Factors—Risks Relating to Our Proposed Acquisitions" and "The Transactions," each included in this prospectus supplement.

The Duke Midwest Acquisition

        We will acquire the Duke Midwest Assets for a purchase price of $2.8 billion in cash, subject to certain adjustments. The Duke Midwest Acquisition is subject to receipt of FERC approval, HSR approval and other customary closing conditions. The Duke Midwest Assets are comprised of (1) 100% ownership interests in five natural gas-fired power facilities located in Ohio, Pennsylvania and Illinois, (2) 100% ownership interest in an oil-fired power facility located in Ohio, (3) partial interests in five coal-fired power facilities located in Ohio and (4) Duke's retail energy business. The facilities operate in the PJM wholesale market, have a diversified fuel mix with baseload coal-fired units, as well as combined cycle and peaking natural gas-fired units, and have an aggregate net generating capacity of approximately 6,200 MW. We will operate two of the five coal-fired plants for which we will obtain partial ownership interests, the Miami Fort and Zimmer facilities.

        Duke Energy Retail Sales serves retail electric and gas customers in Ohio with energy and energy services at competitive rates. Duke Energy Retail Sales serves approximately 25% of switched load within Duke Ohio's service territory, which is the largest share of opt-in mass market customers in that territory, and provides approximately 7 TWh of customer load primarily in Ohio.

        The following table sets forth certain information with respect to the generating facilities:

The ERC and Brayton Acquisitions

        We will acquire ERC and Brayton for approximately $3.25 billion in cash and $200 million of our common stock, subject to certain conditions and adjustments. The ERC and Brayton Acquisitions are subject to receipt of FERC approval, HSR approval and other customary closing conditions. ERC owns (1) 100% interests in five combined cycle gas turbine facilities in Connecticut, Massachusetts and Pennsylvania, (2) 100% interests in two gas and oil fired peaking facilities in Ohio, (3) 100% interests in one coal-fired facility in Illinois and (4) a 49.5% interest in one natural gas-fired peaking facility in Illinois. Brayton owns a 100% interest in the Brayton Point coal facility in Somerset, MA, which is scheduled to be retired in June 2017. The ECP Assets consist primarily of highly efficient and clean-burning natural gas-fired facilities and have an aggregate net generating capacity of approximately 6,300 MW.

        The following table sets forth certain information with respect to the generating facilities:

Combined Company Power Generation Portfolio

        The following table illustrates the Combined Company operating generation facilities:

Strategic Evaluation of California Assets

        Recently, we began a strategic evaluation of our California assets including Moss Landing, Oakland and Morro Bay. We will continue operating and investing in these assets under normal course during this evaluation.

Power Prices and Spark Spreads

        Our financial results are significantly impacted by fuel and commodity prices. Other factors impacting our financial results include market structure and prices for electric energy, capacity and ancillary services, including pricing at our plant locations relative to pricing at their respective trading hubs, the volatility of fuel and electricity prices, transportation and transmission logistics, weather conditions and plant performance.

        The relationship between electricity prices and prices for natural gas and coal, commonly referred to as the "spark spread" and "dark spread," respectively, impacts the margin we earn on the electricity we generate. In particular, we benefit from rising commodity prices. For example, an increase in natural gas prices of $1/MMBTu (net of heat rate contraction) can have a positive material impact on our earnings, as can rising power prices and spark spreads.

        Since the announcement of the Acquisitions, power prices and spark spreads have increased in 2015 and 2016, by approximately 8% to 31% in ISO-NE, by approximately 4% to 11% in PJM and by approximately 3% to 5% in MISO, although there can be no assurances as to how long these price increases may be maintained, if at all. In the six months ended June 30, 2014, and for the year ended December 31, 2013, we, the Duke Midwest Assets and the ECP Assets generated 6.7 million MWh and 11.3 million MWh in ISO-NE, 23.0 million MWh and 45.0 million MWh in PJM and 22.1 million MWh and 45.7 million MWh in MISO, in each case, respectively.

Financing Transactions

        In addition to this offering, we expect to obtain additional financing for the Acquisitions as described below. We cannot assure you that we will complete any of the Financing Transactions on the terms contemplated by this prospectus supplement or at all.

Mandatory Convertible Preferred Stock Offering

        Concurrently with this offering, we are offering, by means of a separate prospectus supplement, 4,000,000 shares of our Mandatory Convertible Preferred Stock, plus up to 600,000 additional shares of Mandatory Convertible Preferred Stock that the underwriters of the Mandatory Convertible Preferred Stock Offering have the option to purchase from us, in each case, at a public offering price of $100.00 per share. For a description of certain of the expected terms of the Mandatory Convertible Preferred Stock, see "Description of Certain Indebtedness and Other Obligations—Dynegy's Mandatory Convertible Preferred Stock". This prospectus supplement is not an offer to sell or a solicitation of an offer to buy the shares of the Mandatory Convertible Preferred Stock.

Debt Financing

        Subsequent to this offering and the Mandatory Convertible Preferred Stock Offering, we expect to obtain approximately $5.1 billion of debt financing in connection with the financing of the Acquisitions. See "The Transactions" and "Description of Indebtedness and Other Obligations," included in this prospectus supplement. This prospectus supplement is not an offer to sell or a solicitation of an offer to buy any debt being sold or placed in the Debt Financing.

        The completion of this offering is not contingent on the completion of the Debt Financing, and the Debt Financing is not contingent on the completion of this offering or the consummation of either or both of the Acquisitions. Accordingly, even if one or both of the Acquisitions or Financing Transactions are not completed, the common stock sold in this offering will remain outstanding.

        We will be required to redeem all of the debt borrowed if neither of the Acquisitions is consummated. If one or the other of the Acquisitions is not consummated, we will be required to redeem the debt borrowed in connection with such contemplated but unconsummated Acquisition.

        In the event we are unable to obtain such debt financing prior to the closing of the Acquisitions, we have obtained committed financing in the form of senior unsecured bridge loans as described below.

        In connection with entering into each of the Duke Midwest Purchase Agreement and the ERC Purchase Agreements, we obtained $2.8 billion and $3.55 billion, respectively, bridge loan commitments to ensure financing for the Acquisitions. Although we do not expect to make any borrowings under the bridge loan commitments, there can be no assurance that such borrowings will not be made. In that regard, we may be required to borrow under the bridge loan commitments if any of the other Financing Transactions is not completed or generates significantly less net proceeds than contemplated by this prospectus supplement. See "Description of Certain Indebtedness and Other Obligations," included in this prospectus supplement.

Strategic Rationale for the Acquisitions

        We believe that the Acquisitions will create significant value for stockholders and offer numerous strategic benefits, including:

•

increasing our presence in the well-structured, highly-desirable PJM and ISO-NE markets through the addition of approximately 12,500 MW of high quality coal and gas assets in favorable locations with strong operational track records;



•

increasing the visibility and predictability of our gross margin from PJM and ISO-NE capacity payments;



•

expanding our retail presence in PJM and providing additional generation in the ComEd region of PJM to complement our existing retail business in that market;

•

enabling the company to leverage its existing infrastructure to reduce the overhead costs per MWh by approximately 35%; and



•

providing opportunities for meaningful synergies, including at least $40 million in cost reductions per year, $200 million in balance sheet efficiencies and nearly $500 million in present value cash tax savings. In order to achieve these synergies, we expect to incur a cost of approximately $40 million.

        We believe that these strategic benefits significantly enhance our competitive profile as a leading U.S. independent power producer and should drive increased profitability while simultaneously reducing the risk profile of the company. We believe the diversity of our portfolio should reduce our operating risk profile and reliance on any single market because it provides a balanced mix of both intrinsic and extrinsic value throughout different market cycles, weather patterns and commodity environments. The breadth of our fleet will also mitigate asset concentration risk by reducing the reliance on any single asset for our overall financial performance. Furthermore, we believe our scale and employment of best practices improve our business development opportunities through well-established industry relationships, and a deep understanding of regional power market dynamics.

Competitive Strengths

        We will have the following competitive strengths following the closing of the Acquisitions:

        Strong Portfolio Balance.    The Combined Company will have a well-diversified portfolio of assets with approximately 51%, 48% and 1% of total generation being fueled by natural gas, coal and oil, respectively, in multiple markets, including MISO, PJM, NYISO, ISO-NE and CAISO. The balance of the portfolio not only provides significant earnings potential and earnings diversity, but also risk mitigation as a greater proportion of the company's earnings are driven by visible capacity payments and the coal and gas fleets tend to partially hedge one another as power and fuel prices change.

        Strong Cash Flow Profile.    The high quality of the acquired assets, their favorable locations and strong capacity factors are expected to result in significant earnings and cash flows which may be used to further strengthen the Combined Company's balance sheet and liquidity position, make investments in existing assets, fund growth opportunities or support other high value capital allocation initiatives.

        Structured Markets.    The Acquisitions will increase our market share in the highly desirable PJM and ISO-NE markets. Both of these markets feature forward capacity, liquid energy and ancillary services markets that provide competitive pricing. Each of our markets benefits from mature regulatory frameworks supportive of existing generation, increasingly attractive market fundamentals and significant challenges to new construction including rising construction costs, more stringent siting and permitting processes and greater scrutiny of environmental impacts. Of the approximately 12,500 MW being acquired, 5,053 MW are modern combined cycle natural gas plants and 3,793 MW are environmentally compliant coal generation plants.

        Improved Scale.    The Acquisitions complement the current size and scale of our existing power generation platform. After the Acquisitions, we expect to be among the largest domestic publicly traded independent power producers, with approximately 26,000 MW of capacity, and benefit from significant diversification in terms of technology, fuel type and dispatch. The Combined Company generation fleet is geographically diversified with assets located in several power markets that feature forward capacity markets, mature regulatory frameworks supportive of existing generation, increasingly attractive market fundamentals, and low risk of becoming overbuilt because of the significant challenges to new construction. Our fleet also has a high degree of dispatch diversity consisting of baseload coal operations, some of the most efficient combined cycle assets that effectively operate in baseload mode because they are located in capacity-constrained regions, load-following capacity and peaking units. Further, the addition of Duke Energy Retail Sales will expand our retail business into three new competitive retail markets (Ohio,

Pennsylvania and Michigan). The ECP Assets and Duke Midwest Assets will provide load following generation to support the retail business.

Business Strategies

        Our business strategy is to create value through the optimization of our generation facilities, cost structure and financial resources.

        Customer Focus.    Our commercial outreach focuses on the needs of the customers and constituents we serve, the end-use and wholesale customer, our market channel partners and the government agencies and regulatory bodies that represent the public interest. The insight provided through these relationships will drive decisions that meet customer needs while optimizing the value of our business.

        Currently, our commercial strategy seeks to optimize the value of our assets by locking in near-term cash flow while preserving the ability to capture higher values long-term as power markets improve. We may hedge portions of the expected output from our facilities with the goal of stabilizing near-term earnings and cash flow while preserving upside potential should commodity prices or market factors improve. Our wholesale origination and trading and retail marketing teams are responsible for implementation of this strategy. These teams provide access to a broad portfolio of customers with varying energy and capacity requirements. There is a significant risk reduction effect from linking our generation to our customer load which reduces the need to purchase financial hedging products in the market.

        Our wholesale origination efforts focus on marketing energy and services through structured transactions that are designed to meet our customers' operating, financial and risk requirements while simultaneously compensating Dynegy appropriately for the products and services delivered. Additionally, we seek to capture the intrinsic and extrinsic value of our generation portfolios. We utilize a wide range of products and contracts such as tolling agreements, fuel supply contracts, capacity auctions, bilateral capacity contracts, power and natural gas swap agreements, power and natural gas options and other financial instruments. The retail marketing effort focuses on offering end-use customers energy products that range from fixed price and full requirements to flexible price and volume structures. Our goal is to deliver value beyond price by leveraging our experience in the energy markets and sharing our expertise to help customers make sound energy decisions. Establishing and maintaining strong relationships with retail energy channel partners is another key focus where personal service and transparent communication further build our current and future retail brands as trusted suppliers. Our objective is to maximize the benefit to both Dynegy and our current and future customers by linking our generation to the load we serve.

        Dynegy operates in a complex and highly-regulated environment with multiple federal, state and local stakeholders, such as legislators, government agencies, industry groups, consumers and environmentalists. These stakeholders are important partners and exhibit influence over regulators and their decisions. Dynegy works with these stakeholders to encourage reasonable regulations that increase shareholder value through driving revenue and containing costs. Our regulatory strategy includes a continuous process of advocacy, visibility, education and building alliances. We also focus on the key issues that most affect our business. The ultimate goal is to find solutions that provide adequate cost recovery and incentivize investment, while providing safe, reliable, cost-effective and environmentally-compliant generation for the communities in which we operate.

        Continuous Improvement.    We have strong plant operations and are committed to operating all of our facilities in a safe, reliable, cost-efficient and environmentally compliant manner. We have dedicated significant resources toward these priorities with approximately $1 billion invested since 2005 in our Coal segment for environmental compliance initiatives to meet contractual obligations and state and federal environmental standards. In addition, we continue to invest across all segments to maintain and improve the safety, reliability and efficiency of the fleet. The Acquisitions highlight our commitment to operating safe, reliable, cost efficient and environmentally compliant power generation facilities. The facilities

comprising the Duke Midwest Assets and the ECP Assets have benefited from regular investment, preventative maintenance and rigorous inspection programs. In addition, we will continue to capitalize on our centralized operations support function with the primary focus of instilling various cost and operating best practices across the fleets in the areas of safety procurement, engineering and outage management.

        We continue to employ our cost and performance improvement initiative, known as PRIDE, which is designed to drive recurring cash flow benefits by optimizing our cost structure, implementing company-wide process and operating improvements, and improving balance sheet efficiency. Our PRIDE program has targeted $135 million in earnings and improvements and $165 million in balance sheet improvements during the 2014-2016 timeframe.

        Capital Structure Management and Allocation.    The power industry is a cyclical commodity business with significant price volatility requiring considerable ongoing capital investment. As such, it is imperative to build and maintain a balance sheet with manageable debt levels supported by a flexible and diverse liquidity program. Our ongoing capital allocation priorities, first and foremost, are to maintain an appropriate leverage and liquidity profile and to make the necessary capital investments to maintain the safety and reliability of our fleet and to comply with environmental rules and regulations. Additional capital allocation options that are evaluated include investments in our existing portfolio, potential acquisitions and returning capital to shareholders. Capital allocation decisions are based on the alternatives that provide the highest risk-adjusted rates of return.

        We continue to focus on building a diverse liquidity program to support our ongoing operations and commercial activities. This includes maintaining adequate cash balances, expanding our first lien collateral program to include additional hedging counterparties and having in place sufficient committed lines of credit to support our ongoing liquidity needs. We will continue to evaluate other measures to best manage our balance sheet as well as seek additional sources of liquidity.

Industry Overview

        The power industry is one of the largest industries in the United States and impacts nearly every aspect of our economy, with an estimated end-user market of approximately $372 billion in power sales in 2013. In 2013, total generation in the United States reached approximately 4 million Gigawatt hours ("GWh"), composed of 27.4% natural gas, 39.1% coal, 19.4% nuclear, 12.8% renewables, and 1.3% other. Historically, the power generation industry has been largely characterized by electric utility monopolies producing electricity from old, less efficient, high-cost generating facilities selling to a captive customer base. Over the last 25 years, however, industry trends and regulatory initiatives, culminating with deregulation in the late 1990s and early 2000s, have transformed the market into a more competitive one where end-users purchase electricity from a variety of suppliers, including non-utility generators, power marketers, public utilities and others. Although models and rules for competition differ from state to state, all markets in which we operate have some form of wholesale market competition.

        The advancing age of the U.S. power generation fleet could present system reliability challenges as the economic efficiency of operating older plants declines over time. More than half of the U.S. power generation fleet is at least 30 years old, with the average age of the U.S. coal fleet being approximately 44 years old. Retirement decisions may also be affected by the cost of compliance with anticipated and existing state and federal environmental regulations. Further, although coal and oil generation assets currently comprise a large portion of planned retirements, baseload nuclear generation has also come under increased pressure. Since 2010, 4.8 GW of nuclear capacity has been retired or is in the process of retiring in the United States and there is a potential for further retirements as several nuclear generators have expressed concerns over maintenance and repair costs, declining profitability in a lower gas price environment and increasing regulatory compliance costs. The anticipated retirement of power generation resources would significantly alter the composition of the existing power supply. As resources are retired,

higher cost units will be dispatched earlier and more often to meet demand, leading to higher power prices, particularly during winter and summer months when the demand for power is greatest.

        We believe that increasing demand and the need to replace old and less efficient generation facilities will create a significant need for additional power generating capacity throughout the United States. In our view, these factors combined with recent restructuring legislation provide an attractive environment in the United States for an independent power producer like us with existing generation assets and a history of successfully developing, acquiring and operating power generation facilities.

Our Markets and Segments

        Currently, Dynegy's largest market is MISO. The Acquisitions will significantly increase our geographic diversity in the PJM and ISO-NE markets.

•

MISO Market.  The MISO market includes all of Wisconsin and portions of Michigan, Kentucky, Indiana, Illinois, Missouri, Arkansas, Mississippi, Texas, Louisiana, Iowa, Minnesota, North Dakota, South Dakota, Montana and Manitoba, Canada. The MISO energy market is designed to ensure that all market participants have open-access to the transmission system on a non-discriminatory basis. MISO, as an independent RTO, maintains functional control over the use of the transmission system to ensure transmission circuits do not exceed their secure operating limits and become overloaded. MISO operates day-ahead and real-time energy markets using a LMP system which calculates a price for every generator and load point within MISO. This market is transparent, allowing generators and load serving entities to see real-time price effects of transmission constraints and the impacts of congestion at each pricing point. An independent market monitor is responsible for evaluating the performance of the markets and identifying conduct by market participants or MISO that may compromise the efficiency or distort the outcome of the markets. In the future, the potential retirement of marginal MISO coal capacity due to poor economics or expected environmental mandates and confirmed future capacity exports from MISO to PJM could affect MISO capacity and energy pricing.



•

PJM Market.  PJM is the largest domestic and most liquid wholesale energy market, serving more than 61 million customers across more than 62,000 miles of transmission lines with over 1,100 generation sources. The PJM market is characterized by upward trends in energy and capacity prices, increasing retirements of coal generation assets, uncertainty of new construction projects and visible multi-year capacity revenue. The PJM market includes all or parts of Delaware, Illinois, Indiana, Kentucky, Maryland, Michigan, New Jersey, North Carolina, Ohio, Pennsylvania, Tennessee, Virginia, West Virginia and the District of Columbia. PJM administers markets for wholesale electricity and provides transmission planning for the region, utilizing the LMP system. PJM operates day-ahead and real-time markets into which generators can bid to provide energy and ancillary services. PJM also administers markets for capacity. An independent market monitor continually monitors PJM markets to ensure a robust, competitive market and to identify any improper behavior by any entity. PJM implemented a forward capacity auction in 2007, the RPM, which established long-term markets for capacity. PJM recently announced several initiatives to improve the effectiveness of the forward capacity market including additional limitations on imports and demand response, which we expect to increase the value of existing power generators. Reserve margins are also expected to tighten following substantial planned retirements due to the ongoing cost of environmental compliance. The Acquisitions will significantly increase our operating capacity to approximately 6% of the total generating capability in the PJM market.



•

ISO-NE Market.  The ISO-NE market, serving the New England region, supplies electricity to 6.5 million households and businesses through 8,500 miles of transmission lines with approximately 350 power generation sources. The ISO-NE market is characterized by high clearing prices, a shortage of existing power generation resources, a lack of new projects under construction and a

significant amount of current power generation assets at risk of retirement. The ISO-NE market includes the six New England states of Vermont, New Hampshire, Massachusetts, Connecticut, Rhode Island and Maine. ISO-NE also dispatches power plants to meet system energy and reliability needs and settles physical power deliveries at LMP. Energy prices vary among the participating states in ISO-NE, and are largely influenced by transmission constraints and fuel supply. This market is also benefitting from favorable supply and demand dynamics in the form of significant capacity retirements, the phase-out of nuclear power generation and the lack of new construction. The Acquisitions will significantly increase our operating capacity to more than 10% of the total generating capability in the ISO-NE market.

        Our existing generation assets are divided into Coal, IPH and Gas segments. Our coal segment is comprised of four operating coal-fired power generation facilities in Illinois with a total generating capacity of 2,980 MW. Our IPH segment is comprised of five operating coal-fired power generation facilities located in Illinois with a total owned generating capacity of 4,062 MW. Our Coal and IPH segments are generally concentrated within the MISO market. The Duke Midwest Assets include partial interests in five coal-fired power generation facilities with a total net generating capacity of 2,701 MW. The ECP Assets include two coal facilities with a total net generating capacity of 2,601 MW, of which 1,493 MW relate to Brayton Point which is scheduled to be retired from service in June 2017.

        Our Gas segment is comprised of six operating natural gas-fired power generation facilities located in California, Illinois, Pennsylvania, New York, and Maine and one fuel-oil fired power generation facility located in California, totaling 6,078 MW of electric generating capacity. Our Gas segment operates in the PJM, NYISO, ISO-NE and CAISO markets. The Duke Midwest Assets include 100% interests in six natural gas/oil-fired facilities with a total net generating capacity of 3,538 MW. The ECP Assets include five natural gas-fired power generation facilities, two natural gas and oil facilities in Ohio, and partial interests in one gas-fired power generation facility in Illinois, with a total net generating capacity of 3,748 MW.

Corporate Information

        Our principal executive offices are located at 601 Travis, Suite 1400, Houston, Texas 77002. Our telephone number is (713) 507-6400 and we have a website accessible at www.dynegy.com. The information posted on our website is not incorporated into this prospectus supplement and is not part of this prospectus supplement.

THE OFFERING

        The summary below contains basic information about this offering. It does not contain all of the information that is important to you. You should read this prospectus supplement and the accompanying prospectus and the documents incorporated and deemed to be incorporated by reference in this prospectus supplement and the accompanying prospectus carefully before making an investment decision.

        The number of shares of our common stock to be outstanding immediately after this offering is based on 22,500,000 shares to be issued in this offering and 100,382,015 shares outstanding as of October 2, 2014, and excludes:

•

up to                    shares of our common stock (up to                    shares of common stock if the underwriters in the Mandatory Convertible Preferred Stock Offering exercise their option to purchase additional Mandatory Convertible Preferred Stock in full), in each case subject to anti-dilution, make-whole and other adjustments, that would be issuable upon conversion of

Mandatory Convertible Preferred Stock issued in the Mandatory Convertible Preferred Stock Offering;

•

2,892,047 shares of our common stock available for future grant under our existing equity incentive plans as of October 2, 2014;



•

17,287,192 shares of our common stock issuable upon exercise of options, warrants and performance units, outstanding as of October 2, 2014; and



•

the common stock placement to ECP.

        Unless otherwise specified or the context requires otherwise, information in this prospectus supplement assumes that (1) the option we have granted to the underwriters in this offering to purchase 3,375,000 additional shares of common stock and the option we have granted to the underwriters in the Mandatory Convertible Preferred Stock Offering to purchase 600,000 additional shares of Mandatory Convertible Preferred Stock, in each case, are not exercised, (2) the Mandatory Convertible Preferred Stock will not be redeemed if either or both of the Acquisitions are not consummated and (3) we elect to pay any and all dividends with respect to the Mandatory Convertible Preferred Stock in cash.

SUMMARY HISTORICAL CONSOLIDATED FINANCIAL DATA OF DYNEGY

        The selected financial information presented below as of December 31, 2012 and 2013 and for the year ended December 31, 2011, the period from January 1 through October 1, 2012, the period from October 2, 2012 through December 31, 2012, and the year ended December 2013 was derived from, and is qualified by reference to, our audited consolidated financial statements, including the notes thereto, which are incorporated by reference in this prospectus supplement. The selected financial information presented below for the years ended December 31, 2009 and 2010 was derived from, and is qualified by reference to, our audited consolidated financial statements, including the notes thereto, which are not incorporated by reference in this prospectus supplement. The selected financial information as of June 30, 2014 and for the six months ended June 30, 2013 and 2014, was derived from, and is qualified by reference to, our consolidated financial statements, including the notes thereto, which are incorporated by reference in this prospectus supplement.

        This information is only a summary. You should read the data set forth in the table below in conjunction with the section entitled "Selected Financial Data" in our Annual Report on Form 10-K for the fiscal year ended December 31, 2013 and the section entitled "Part I. Financial Information" in our Quarterly Report on Form 10-Q for the quarter ended June 30, 2014, which are incorporated by reference in this prospectus supplement. As a result of the application of fresh-start accounting as of the October 1, 2012, following our reorganization, the financial statements on or prior to October 1, 2012 are not comparable with the financial statements after October 1, 2012. References to "Successor" refer to Dynegy after October 1, 2012, after giving effect to the application of fresh-start accounting. References to "Predecessor" refer to Dynegy on or prior to October 1, 2012.

SUMMARY HISTORICAL CONSOLIDATED FINANCIAL DATA OF DUKE MIDWEST ASSETS

        The following table sets forth summary historical consolidated financial data for the Duke Midwest Assets as of and for the periods ended on the dates indicated below. We have derived the summary consolidated financial data as of and for the years ended December 31, 2011, 2012 and 2013 from the audited consolidated financial statements incorporated by reference into this prospectus supplement. We have derived the summary consolidated financial data as of June 30, 2014 and for the six-months ended June 30, 2013 and 2014, from the unaudited financial statements incorporated by reference into this prospectus supplement. The unaudited financial statements were prepared on the same basis as the audited financial statements and, in the opinion of management, include all adjustments, consisting only of normal, recurring adjustments necessary for a fair statement of the information set forth therein.