<Page> UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 ------------------------------------------------ FORM 8-K/A (Amendment No. 1) CURRENT REPORT PURSUANT TO SECTION 13 OR 15 (d) OF THE SECURITIES AND EXCHANGE ACT OF 1934 DATE OF REPORT (DATE OF EARLIEST EVENT REPORTED) DECEMBER 13, 2001 ------------------------------------------------ NORTHWESTERN CORPORATION (Exact name of registrant as specified in charter) DELAWARE -------- (State or other jurisdiction of incorporation) 0-692 46-0172280 ------- ---------- (Commission File Number) (IRS Employer Identification Number) 125 SOUTH DAKOTA AVENUE, SIOUX FALLS, SD 57104 ---------------------------------------------- (Address of registrant's principal executive office) Registrant's telephone number, including area code: (605) 978-2908 -------------- Not Applicable ------------------------------------------------------- (Former Name or Former Address, if Changed Since Last Report <Page> The sole purpose of this Amendment No. 1 to our Current Report on Form 8-K filed with the Securities and Exchange Commission (the "SEC") on December 12, 2001 is to file Exhibit 10.1, Exhibit 10.2, Exhibit 10.3, Exhibit 10.4, Exhibit 10.5, Exhibit 10.6, Exhibit 10.7, Exhibit 10.8 and Exhibit 23. ITEM 7. FINANCIAL STATEMENTS, PRO FORMA FINANCIAL INFORMATION AND EXHIBITS (c) EXHIBITS EXHIBIT NUMBER TITLE - ------- ----- 10.1 Comprehensive Employment Agreement and Investment Program for Merle D. Lewis, dated as of June 1, 2000. 10.2 Comprehensive Employment Agreement and Equity Plan Participation Program for Richard R. Hylland, dated as of March 1, 2001. 10.3 Comprehensive Employment Agreement and Equity Plan Participation Program for Daniel K. Newell, dated as of March 1, 2001. 10.4 Comprehensive Employment Agreement and Equity Plan Participation Program for Michael J. Hanson, dated as of March 1, 2001. 10.5 Comprehensive Employment Agreement and Equity Plan Participation Program for Walter A. Bradley, III, dated as of March 1, 2001. 10.6 Comprehensive Employment Agreement and Equity Plan Participation Program for Kipp D. Orme, dated as of March 1, 2001. 10.7 Comprehensive Employment Agreement and Equity Plan Participation Program for Eric R. Jacobsen, dated as of March 1, 2001. 10.8 Supplemental Income Security Plan for Directors, Officers and Managers, as amended and restated effective July 1, 1999. 23 Consent of PricewaterhouseCoopers LLC. 99.1 * Unaudited Combined Financial Statements of the Utility as of and for the nine months ended September 30, 2001 and Audited Combined Financial Statements of the Utility as of December 31, 2000 and 1999 and for each of the years in the three-year period ended December 31, 2000. 99.2 * Unaudited Pro Forma Combined Financial Information of NorthWestern Corporation as of and for the nine months ended September 30, 2001 and for the year ended December 31, 2000. 99.3 * Unaudited Pro Forma Combined Financial Data of the Utility as of and for the nine months ended September 30, 2001 and for the year ended December 30, 2000. * Previously filed. <Page> SIGNATURES Pursuant to the requirements of the Securities and Exchange Act of 1934, the Registrant has duly caused this report to be signed on its behalf by the undersigned thereunto duly authorized. NorthWestern Corp. (Registrant) By: /s/ Kipp D. Orme ------------------------------ Kipp D. Orme Vice President--Finance and Chief Executive Officer December 13, 2001 <Page> EXHIBIT INDEX EXHIBIT NUMBER TITLE - ------- ----- 10.1 Comprehensive Employment Agreement and Investment Program for Merle D. Lewis, dated as of June 1, 2000. 10.2 Comprehensive Employment Agreement and Equity Plan Participation Program for Richard R. Hylland, dated as of March 1, 2001. 10.3 Comprehensive Employment Agreement and Equity Plan Participation Program for Daniel K. Newell, dated as of March 1, 2001. 10.4 Comprehensive Employment Agreement and Equity Plan Participation Program for Michael J. Hanson, dated as of March 1, 2001. 10.5 Comprehensive Employment Agreement and Equity Plan Participation Program for Walter A. Bradley, III, dated as of March 1, 2001. 10.6 Comprehensive Employment Agreement and Equity Plan Participation Program for Kipp D. Orme, dated as of March 1, 2001. 10.7 Comprehensive Employment Agreement and Equity Plan Participation Program for Eric R. Jacobsen, dated as of March 1, 2001. 10.8 Supplemental Income Security Plan for Directors, Officers and Managers, as amended and restated effective July 1, 1999. 23 Consent of PricewaterhouseCoopers LLC. 99.1 * Unaudited Combined Financial Statements of the Utility as of and for the nine months ended September 30, 2001 and Audited Combined Financial Statements of the Utility as of December 31, 2000 and 1999 and for each of the years in the three-year period ended December 31, 2000. 99.2 * Unaudited Pro Forma Combined Financial Information of NorthWestern Corporation as of and for the nine months ended September 30, 2001 and for the year ended December 31, 2000. 99.3 * Unaudited Pro Forma Combined Financial Data of the Utility as of and for the nine months ended September 30, 2001 and for the year ended December 30, 2000. * Previously filed.