<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0306</schemaVersion>

    <documentType>4/A</documentType>

    <periodOfReport>2013-07-31</periodOfReport>

    <dateOfOriginalSubmission>2013-10-24</dateOfOriginalSubmission>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001490286</issuerCik>
        <issuerName>TORTOISE MLP FUND, INC.</issuerName>
        <issuerTradingSymbol>NTG</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001097362</rptOwnerCik>
            <rptOwnerName>SUN LIFE FINANCIAL INC</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>SUN LIFE ASSURANCE CO OF CANADA</rptOwnerStreet1>
            <rptOwnerStreet2>150 KING STREET WEST SUITE 1400</rptOwnerStreet2>
            <rptOwnerCity>TORONTO</rptOwnerCity>
            <rptOwnerState>A6</rptOwnerState>
            <rptOwnerZipCode>M5H 1J9</rptOwnerZipCode>
            <rptOwnerStateDescription>ONTARIO, CANADA</rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>4.29% Senior Notes Series D due 12/15/2020</value>
            </securityTitle>
            <transactionDate>
                <value>2013-07-31</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>1000000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>1079960</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>13000000</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See footnote.</value>
                    <footnoteId id="F1"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">$12,000,000 directly owned by Sun Life Assurance Company of Canada, a wholly-owned subsidiary of Sun Life Financial Inc.and $1,000,000 directly owned by Sun Life and Health Insurance Company (U.S.), an indirectly wholly-owned subsidiary of Sun Life Financial Inc.</footnote>
    </footnotes>

    <remarks>Sun Life and Health Insurance Company (U.S.), an indirectly wholly-owned subsidiary of Sun Life Financial Inc., purchased $1,000,000 of Series D Notes from Sun Life Insurance and Annuity Company of New York, then also an indirectly wholly-owned subsidiary of Sun Life Financial Inc.  Previously incorrectly reported as disposed of via transfer to Delaware Life Holdings LLC on August 2, 2013 of 100% of the stock of Sun Life Assurance Company of Canada (U.S.), the owner of 100% of the stock of Sun Life Insurance and Annuity Company of New York. At this time, Sun Life and Health Insurance Company (U.S.) does not have a CIK or CCC number.</remarks>

    <ownerSignature>
        <signatureName>Stephen C. Peacher, signer for Sun Life Financial Inc. and Sun Life and Health Insurance Company (U.S.)</signatureName>
        <signatureDate>2013-11-21</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>John T. Donnelly, signer for Sun Life Financial Inc. and Sun Life and Health Insurance Company (U.S.)</signatureName>
        <signatureDate>2013-11-21</signatureDate>
    </ownerSignature>
</ownershipDocument>
