<PAGE>




                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D. C. 20549

                           --------------------------

                                    FORM 8-K

                                 CURRENT REPORT

                       Pursuant to Section 13 or 15(d) of
                       The Securities Exchange Act of 1934

                            -------------------------

       Date of Report (Date of earliest event reported): December 1, 2005

                          MERRILL LYNCH DEPOSITOR, INC.
                 (ON BEHALF OF PREFERREDPLUS TRUST SERIES NAI-1)
             (Exact name of registrant as specified in its charter)

            DELAWARE             001-16549                     13-3891329
        (State or other         (Commission                (I. R. S. Employer
        jurisdiction of         File Number)               Identification No.)
         incorporation)

     WORLD FINANCIAL CENTER,                                     10080
       NEW YORK, NEW YORK                                     (Zip Code)
      (Address of principal
       executive offices)

                           --------------------------

       Registrant's telephone number, including area code: (212) 449-1000

                      INFORMATION TO BE INCLUDED IN REPORT


SECTION 1.        REGISTRANT'S BUSINESS AND OPERATIONS

                  Not applicable.

SECTION 2.        FINANCIAL INFORMATION

                  Not applicable.

SECTION 3.        SECURITIES AND TRADING MARKETS

                  Not applicable.

SECTION 4.        MATTERS RELATED TO ACCOUNTANTS AND FINANCIAL STATEMENTS

                  Not applicable.

SECTION 5.        CORPORATE GOVERNANCE AND MANAGEMENT

                  Not applicable.
<PAGE>

SECTION 6.        ASSET-BACKED SECURITIES

                  Not applicable.

SECTION 7.        REGULATION FD

                  Not applicable.

SECTION 8.        OTHER EVENTS

     ITEM 8.01    OTHER EVENTS

                  99.1     Distribution to holders of the PreferredPLUS Trust
                           Certificates Series NAI-1 on December 1, 2005.

                           For information with respect to the underlying
                  securities held by PreferredPLUS Trust Series NAI-1, please
                  refer to News Corporation's (Commission file number 001-32352)
                  periodic reports, including annual reports on Form 10-K,
                  quarterly reports on Form 10-Q and current reports on Form
                  8-K, and other information on file with the Securities and
                  Exchange Commission (the "SEC"). You can read and copy these
                  reports and other information at the public reference
                  facilities maintained by the SEC at Room 1580, 100 F Street,
                  NE, Washington, D.C. 20549. You may obtain copies of this
                  material for a fee by writing to the SEC's Public Reference
                  Section of the SEC at 100 F Street, NE, Washington, D.C.
                  20549. You may obtain information about the operation of the
                  Public Reference Room by calling the SEC at 1-800-SEC-0330.
                  You can also access some of this information electronically by
                  means of the SEC's website on the Internet at
                  http://www.sec.gov, which contains reports, proxy and
                  information statements and other information that the
                  underlying securities issuer has filed electronically with the
                  SEC.

                           Although we have no reason to believe the
                  information concerning the underlying securities, the
                  guarantee, the underlying securities issuer or the
                  underlying securities guarantor contained in the
                  underlying securities issuer's Exchange Act reports
                  is not reliable, neither the depositor nor the
                  trustee participated in the preparation of such
                  documents or made any due diligence inquiry with
                  respect to the information provided therein. No
                  investigation with respect to the underlying
                  securities issuer or any underlying securities
                  guarantor (including, without limitation, no
                  investigation as to their respective financial
                  condition or creditworthiness) or of the underlying
                  securities or the guarantee has been made. You should
                  obtain and evaluate the same information concerning
                  the underlying securities issuer and each underlying
                  securities guarantor as you would obtain and evaluate
                  if you were investing directly in the underlying
                  securities or in other securities issued by the
                  underlying securities issuer or each underlying
                  securities guarantor. There can be no assurance that
                  events affecting the underlying securities, the
                  guarantee, the underlying securities issuer or the
                  underlying securities guarantor have not occurred or
                  have not yet been publicly disclosed which would
                  affect the accuracy or completeness of the publicly
                  available documents described above.
<PAGE>


SECTION 9.        FINANCIAL STATEMENTS AND EXHIBITS

    ITEM 9.01     FINANCIAL STATEMENTS AND EXHIBITS


             (a)   Financial statements of business acquired.

                   Not applicable.

             (b)   Pro forma financial information.

                   Not applicable.

             (c)   Shell company transactions.

                   Not applicable.

             (d)   Exhibits.

                   99.1    Trustee's report in respect of the December 1, 2005
                           distribution to holders of the PreferredPLUS Trust
                           Certificates Series NAI-1.



                              SIGNATURES

             Pursuant to the requirements of the Securities Exchange Act of
             1934, the registrant has duly caused this report to be signed
             on its behalf by the undersigned, hereunto duly authorized.


                                      MERRILL LYNCH DEPOSITOR, INC.

Date: December 8, 2005                By:    /s/ Stephan Kuppenheimer
                                              ------------------------------
                                      Name:  Stephan Kuppenheimer
                                      Title: President

<PAGE>

                                  EXHIBIT INDEX


       99.1     Trustee's report in respect of the December 1, 2005
                distribution to holders of the PreferredPLUS Trust
                Certificates Series NAI-1.
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SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D. C. 20549

                           --------------------------

                                    FORM 8-K

                                 CURRENT REPORT

                       Pursuant to Section 13 or 15(d) of
                       The Securities Exchange Act of 1934

                            -------------------------

       Date of Report (Date of earliest event reported): December 1, 2005

                          MERRILL LYNCH DEPOSITOR, INC.
                 (ON BEHALF OF PREFERREDPLUS TRUST SERIES NAI-1)
             (Exact name of registrant as specified in its charter)

            DELAWARE             001-16549                     13-3891329
        (State or other         (Commission                (I. R. S. Employer
        jurisdiction of         File Number)               Identification No.)
         incorporation)

     WORLD FINANCIAL CENTER,                                     10080
       NEW YORK, NEW YORK                                     (Zip Code)
      (Address of principal
       executive offices)

                           --------------------------

       Registrant's telephone number, including area code: (212) 449-1000

                      INFORMATION TO BE INCLUDED IN REPORT

SECTION 1.        REGISTRANT'S BUSINESS AND OPERATIONS

                  Not applicable.

SECTION 2.        FINANCIAL INFORMATION

                  Not applicable.

SECTION 3.        SECURITIES AND TRADING MARKETS

                  Not applicable.

SECTION 4.        MATTERS RELATED TO ACCOUNTANTS AND FINANCIAL STATEMENTS

                  Not applicable.

SECTION 5.        CORPORATE GOVERNANCE AND MANAGEMENT

                  Not applicable.
SECTION 6.        ASSET-BACKED SECURITIES

                  Not applicable.

SECTION 7.        REGULATION FD

                  Not applicable.

SECTION 8.        OTHER EVENTS

     ITEM 8.01    OTHER EVENTS

                  99.1     Distribution to holders of the PreferredPLUS Trust
                           Certificates Series NAI-1 on December 1, 2005.

                           For information with respect to the underlying
                  securities held by PreferredPLUS Trust Series NAI-1, please
                  refer to News Corporation's (Commission file number 001-32352)
                  periodic reports, including annual reports on Form 10-K,
                  quarterly reports on Form 10-Q and current reports on Form
                  8-K, and other information on file with the Securities and
                  Exchange Commission (the "SEC"). You can read and copy these
                  reports and other information at the public reference
                  facilities maintained by the SEC at Room 1580, 100 F Street,
                  NE, Washington, D.C. 20549. You may obtain copies of this
                  material for a fee by writing to the SEC's Public Reference
                  Section of the SEC at 100 F Street, NE, Washington, D.C.
                  20549. You may obtain information about the operation of the
                  Public Reference Room by calling the SEC at 1-800-SEC-0330.
                  You can also access some of this information electronically by
                  means of the SEC's website on the Internet at
                  http://www.sec.gov, which contains reports, proxy and
                  information statements and other information that the
                  underlying securities issuer has filed electronically with the
                  SEC.

                           Although we have no reason to believe the
                  information concerning the underlying securities, the
                  guarantee, the underlying securities issuer or the
                  underlying securities guarantor contained in the
                  underlying securities issuer's Exchange Act reports
                  is not reliable, neither the depositor nor the
                  trustee participated in the preparation of such
                  documents or made any due diligence inquiry with
                  respect to the information provided therein. No
                  investigation with respect to the underlying
                  securities issuer or any underlying securities
                  guarantor (including, without limitation, no
                  investigation as to their respective financial
                  condition or creditworthiness) or of the underlying
                  securities or the guarantee has been made. You should
                  obtain and evaluate the same information concerning
                  the underlying securities issuer and each underlying
                  securities guarantor as you would obtain and evaluate
                  if you were investing directly in the underlying
                  securities or in other securities issued by the
                  underlying securities issuer or each underlying
                  securities guarantor. There can be no assurance that
                  events affecting the underlying securities, the
                  guarantee, the underlying securities issuer or the
                  underlying securities guarantor have not occurred or
                  have not yet been publicly disclosed which would
                  affect the accuracy or completeness of the publicly
                  available documents described above.
SECTION 9.        FINANCIAL STATEMENTS AND EXHIBITS

    ITEM 9.01     FINANCIAL STATEMENTS AND EXHIBITS

             (a)   Financial statements of business acquired.

                   Not applicable.

             (b)   Pro forma financial information.

                   Not applicable.

             (c)   Shell company transactions.

                   Not applicable.

             (d)   Exhibits.

                   99.1    Trustee's report in respect of the December 1, 2005
                           distribution to holders of the PreferredPLUS Trust
                           Certificates Series NAI-1.

                              SIGNATURES

             Pursuant to the requirements of the Securities Exchange Act of
             1934, the registrant has duly caused this report to be signed
             on its behalf by the undersigned, hereunto duly authorized.

                                      MERRILL LYNCH DEPOSITOR, INC.

Date: December 8, 2005                By:    /s/ Stephan Kuppenheimer
                                              ------------------------------
                                      Name:  Stephan Kuppenheimer
                                      Title: President
EXHIBIT INDEX

       99.1     Trustee's report in respect of the December 1, 2005
                distribution to holders of the PreferredPLUS Trust
                Certificates Series NAI-1.