Summary Prospectus by Investment Company - Form 497K

INVESTMENT OBJECTIVE

This Fund seeks capital appreciation and income.

FEES AND EXPENSES OF THE FUND

This table describes the fees and expenses that you may pay if you buy, hold, and sell shares of the Fund. The expenses in the table do not reflect deductions at the separate account level or contract level for any charges that may be incurred under a variable life insurance or variable annuity contract. If these charges were reflected, the fees and expenses in the table would be higher.

Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)

Example

This example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. It assumes that you invest $10,000 in each share class of the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The example also assumes that your investment earns a 5% return each year and that the Fund's operating expenses are exactly as described in the preceding table. If separate account or variable life insurance or variable annuity contract expenses were included, overall expenses would be higher.

Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Portfolio Turnover

The Fund pays transaction costs, such as commissions, when it buys and sells securities (or "turns over" its portfolio). A higher portfolio turnover rate may indicate higher transaction costs. These costs, which are not reflected in annual fund operating expenses or in the example, affect the Fund's performance. During the most recent fiscal year, the Fund's portfolio turnover rate was 29% of the average value of its portfolio.

INVESTMENTS, RISKS, AND PERFORMANCE​

Principal Investment Strategies

Under normal circumstances, the Fund invests at least 80% of its net assets (plus the amount of any borrowings for investment purposes) in sustainable equity securities. Equity securities may include common stock, preferred stock, securities convertible into common or preferred stock, rights, and warrants. The Fund's subadviser, American Century Investment Management, Inc. ("American Century"), currently considers sustainable securities to be those to which the subadviser's proprietary model assigns an ESG score that is in the top three quartiles of the environmental, social, and governance ("ESG") scores the model assigns to all of the securities in the Fund's benchmark, the S&P 500® Index.

The Fund generally invests in large-capitalization companies American Century believes show improving business fundamentals and attractive ESG characteristics compared to their peer

companies, using a proprietary multi-factor model that combines fundamental measures of a stock's value and growth potential with ESG metrics. The model assigns each security a financial metrics score and an ESG score that are combined to create an overall score. American Century currently defines large capitalization companies as companies with capitalizations in the capitalization range of the S&P 500 Index. The Fund may at times have significant exposure to one or more industries or sectors. The Fund may hold a portion of its assets in cash or cash equivalents.

To measure value, American Century may use ratios of stock price-to-earnings and stock price-to-cash flow. To measure growth, American Century may use the rate of growth of a company's earnings and cash flow and changes in its earnings estimates. The model also considers price momentum. American Century arrives at an ESG score by evaluating multiple metrics of each ESG characteristic - environmental, social, and governance. American Century utilizes internal data and research, as well as third party commercial data sources and scoring systems, to evaluate each security's ESG characteristics. To the extent such information is available and relevant for a particular company, American Century will consider, among other things, a company's carbon emission profile, energy and water usage, or waste generation (environmental), a company's employee turnover rates, digital privacy, or worker safety (social), and a company's corporate leadership, including board chair independence and the independence of audit and compensation committees or shareholder rights such as say on pay (governance). If an ESG score is unavailable or incomplete, a security may still be selected for the portfolio if American Century believes it can evaluate the security qualitatively, or if the financial metrics and/or remaining ESG data merit investment. Qualitative review of portfolio securities may include examination of registration statements and other information provided by the company as well as engagement with company management.

Final scores for each security are evaluated on a sector-specific basis, and the Fund seeks to hold securities with the strongest scores in their respective sectors. Using this process, American Century attempts to build a portfolio of stocks that has sustainable competitive advantages, provides better returns without taking on significant additional risk, and maintains a stronger ESG profile than the S&P 500 Index.

Although American Century intends to invest the Fund's assets primarily in U.S. securities, the Fund may invest in securities of foreign companies when these securities meet American Century's standards of selection.

When determining whether to sell a security, American Century considers among other things, a security's price, whether a security's risk parameters outweigh its return opportunities, general market conditions, and whether the security meets its ESG criteria.

Principal Risks

The following are the Principal Risks of the Fund. The value of your investment in the Fund could go down as well as up. You can lose money by investing in the Fund. Certain risks relating to instruments and strategies used in the management of the Fund are placed first. The significance of any specific risk to an investment in the Fund will vary over time, depending on the composition of the Fund's portfolio, market conditions, and other factors. You should read all of the risk information presented below carefully, because any one or more of these risks may result in losses to the Fund.

Style Risk Different investment styles tend to shift in and out of favor, depending on market and economic conditions as well as investor sentiment. If at any time, the market is not favoring the Fund's investment style, the Fund's gains may not be as big as, or its losses may be bigger than, those of other funds using different investment styles.

ESG Risk To the extent the Fund's investments are selected based in whole or in part on ESG factors or other non-financial considerations, the Fund may not invest in certain securities in which it might otherwise invest, and may experience less favorable performance than portfolios that do not consider ESG and other non-financial factors or that consider them differently. The securities of companies with favorable ESG scores may underperform similar companies that do not score as well or may underperform the stock market as a whole. Information developed or analyzed by the Fund's managers or by third parties to determine a company's ESG score may not be complete or accurate, and complete ESG-related information or data may not be available for many companies. Reliance on ESG scores and factors by the Fund's managers may have the effect of prioritizing the Fund's long-term returns over short-term returns.

Equity Securities Risk Although stocks may have the potential to outperform other asset classes over the long term, their prices tend to fluctuate more dramatically over the shorter term. These movements may result from factors affecting individual companies, or from broader influences like changes in interest rates, market conditions, or investor confidence, or announcements of economic, political, or financial information.

Foreign Investment Risk; Emerging Markets Risk; Currency Risk Investments in securities of foreign issuers, securities of companies with significant foreign exposure, and foreign currencies can involve additional risks relating to market, industry, political, regulatory, public health, and other conditions. Political, social, diplomatic, and economic developments, U.S. and foreign government action, or threat thereof, such as the imposition of currency or capital blockages, controls, or tariffs, economic and trade sanctions or embargoes, security trading suspensions, entering or exiting trade or other intergovernmental agreements, or the expropriation or nationalization of assets in a particular country, can cause dramatic declines in certain or all securities with exposure to that country and other countries. Sanctions, or the threat of sanctions, may cause volatility in regional and global markets and may negatively impact the performance of various sectors and industries, as well as companies in other countries, which could have a negative effect on the performance of the Fund. In the event of nationalization, expropriation, confiscation, or other government action, intervention, or restriction, the Fund could lose its entire investment in a particular foreign issuer or country. There may be quotas or other limits on the ability of the Fund (or clients of the Fund's investment adviser or subadviser) to invest or maintain investments in securities of issuers in certain countries. Enforcing legal rights can be more difficult, costly, and limited in certain foreign countries and with respect to certain types of investments, and can be particularly difficult against foreign governments. Because non-U.S. securities are normally denominated and traded in currencies other than the U.S. dollar, the value of the Fund's assets may be affected favorably or unfavorably by changes in currency exchange rates, exchange control regulations, and restrictions or prohibitions on the repatriation of non-U.S. currencies. Income and gains with respect to investments in certain countries may be subject to withholding and other taxes. There may be less information publicly available about a non-U.S.

company than about a U.S. company, and many non-U.S. companies are not subject to accounting, auditing, and financial reporting standards, regulatory framework and practices comparable to those in the U.S. The securities of some non-U.S. companies, especially those in emerging markets, are less liquid and at times more volatile than securities of comparable U.S. companies. Emerging markets securities are subject to greater risks than securities issued in developed foreign markets, including less liquidity, less stringent investor protection and disclosure standards, less reliable settlement practices, greater price volatility, higher relative rates of inflation, greater political, economic, and social instability, greater custody and operational risks, greater risk of new or inconsistent government treatment of or restrictions on issuers and instruments, and greater volatility in currency exchange rates, and are more susceptible to environmental problems. Many emerging market countries are highly reliant on international trade and exports, including the export of commodities. Their economies may be significantly impacted by fluctuations in commodity prices and the global demand for certain commodities. In addition, pandemics and outbreaks of contagious diseases may exacerbate pre-existing problems in emerging market countries with less established health care systems. Frontier markets, a subset of emerging markets, generally have smaller economies and less mature capital markets than emerging markets. As a result, the risks of investing in emerging market countries are magnified in frontier market countries. Frontier markets are more susceptible to having abrupt changes in currency values, less mature markets and settlement practices, and lower trading volumes that could lead to greater price volatility and illiquidity. Non-U.S. transaction costs, such as brokerage commissions and custody costs, may be higher than in the United States. In addition, foreign markets can react differently to market, economic, industry, political, regulatory, geopolitical, public health, and other conditions than the U.S. market.

Large Company Risk Large-capitalization stocks as a group could fall out of favor with the market, causing the Fund's investments in large-capitalization stocks to underperform investments that focus on small- or medium-capitalization stocks. Larger, more established companies may be slow to respond to challenges and may grow more slowly than smaller companies.

Cash Position Risk If the Fund holds a significant portion of its assets in cash or cash equivalents,

its investment returns may be adversely affected and the Fund may not achieve its investment objective.

Convertible Securities Risk Convertible securities are subject to the risks of both debt instruments and equity securities. The price of a convertible security may change in response to changes in price of the underlying equity security, the credit quality of the issuer, and interest rates. In general, the values of convertible securities tend to decline as interest rates rise and to rise when interest rates fall. A convertible security generally has less potential for gain or loss than the underlying equity security.

Liquidity Risk Certain securities may be difficult (or impossible) to sell or certain positions may be difficult to close out at a desirable time and price, and the Fund may be required to hold an illiquid investment that is declining in value, or it may be required to sell certain illiquid investments at a price or time that is not advantageous in order to meet redemptions or other cash needs. Some securities may be subject to restrictions on resale. There can be no assurance that there will be a liquid market for instruments held by the Fund at any time. The Fund may not receive the proceeds from the sale of certain investments for an extended period.

Management and Operational Risk The Fund relies on the manager's investment analysis and its selection of investments to achieve its investment objective, and the Fund is subject to the risk that the manager's assessment of an investment is wrong. There can be no assurance that the Fund will achieve the intended results and the Fund may incur significant losses. The Fund also runs the risk that deficiencies in the investment adviser's, subadviser's, or another service provider's internal systems or controls will cause losses for the Fund or impair Fund operations.

Market Risk The value of the Fund's portfolio securities may decline, at times sharply and unpredictably, as a result of unfavorable market-induced changes affecting particular industries, sectors, or issuers. Stock and bond markets can decline significantly in response to issuer, market, economic, industry, political, regulatory, geopolitical, public health, and other conditions, as well as investor perceptions of these conditions. The Fund is subject to risks affecting issuers, such as management performance, financial leverage, industry problems, and reduced demand for goods or services.

Preferred Stock Risk Like other equity securities, preferred stock is subject to the risk that its value

may decrease based on actual or perceived changes in the business or financial condition of the issuer. In addition, changes in interest rates may adversely affect the value of a preferred stock that pays a fixed dividend. Preferred stocks are also subject to additional risks, such as potentially greater volatility and risks related to deferral, non-cumulative dividends, subordination, liquidity, limited voting rights, and special redemption rights.

Sector Risk The Fund may allocate more of its assets to particular industries or to particular economic, market, or industry sectors than to others. This could increase the volatility of the Fund's portfolio, and the Fund's performance may be more susceptible to developments affecting issuers in those industries or sectors than if the Fund invested more broadly.

Valuation Risk The Fund is subject to the risk of mispricing or improper valuation of its investments, in particular to the extent that its securities are fair valued.

Performance Information

The following bar chart and table provide some indication of the risks of investing in the Fund. The bar chart shows changes in the Fund's performance from year to year for Initial Class shares. The table shows how the Fund's average annual returns for 1, 5, and 10 years compare with those of a broad measure of market performance. The Fund's name and investment strategy changed on April 29, 2022. The performance results shown below would not necessarily have been achieved had the Fund's current investment strategy been in effect for the entire period for which performance results are presented. Performance shown does not reflect the fees and expenses deducted under the variable life insurance or variable annuity contract through which you invest in the Fund. If these amounts were reflected, returns would be less than those shown. Past performance is not necessarily an indication of how the Fund will perform in the future.

Annual Performance

Initial Class Shares

Average Annual Total Returns

(for the periods ended December 31, 2023)

MANAGEMENT

Investment Adviser: MML Investment Advisers, LLC ("MML Advisers")

Subadviser(s): American Century Investment Management, Inc. ("American Century")

Portfolio Manager(s):

Rob Bove is a Portfolio Manager at American Century. He has managed the Fund since April 2022.

Justin Brown, CFA is a Vice President and Portfolio Manager at American Century. He has managed the Fund since April 2022.

Joe Reiland, CFA is a Vice President and Senior Portfolio Manager at American Century. He has managed the Fund since April 2022.

PURCHASE AND SALE OF FUND SHARES

Shares of the Fund are generally available to separate investment accounts of variable life insurance and variable annuity contracts offered by companies such as MassMutual. An investor should consult the company issuing the investor's variable life insurance or variable annuity contract to determine how to make redemptions.

TAX INFORMATION

Dividends and capital gain distributions are paid to the insurance company separate accounts. Variable life insurance and variable annuity contract owners should refer to the variable life insurance or variable annuity product prospectus or consult with their own tax adviser for information regarding the tax consequences of their investment.

PAYMENTS TO INSURANCE COMPANIES AND THEIR AFFILIATES

The Fund is not sold directly to the general public but instead is offered as an underlying investment option for variable insurance contracts. The Fund and its related companies may make payments to the sponsoring insurance company (or its affiliates) or other intermediary for distribution and/or other services. These payments may be a factor that the insurance company considers in including the Fund as an underlying investment option in the variable insurance contract or may create a conflict of interest by influencing the insurance company or other intermediary to recommend the variable insurance contract over another investment. The prospectus (or other offering document) for your variable insurance contract may contain additional information about these payments.