Biwi No.1 is the story of Puja (Karisma Kapoor) and her husband Prem (Salman Khan) whom she marries much against the wishes of her wealthy parents. Prem's fortune changes and he becomes the head of a large advertising firm. His success is attributed to the love and support of Puja. But their idyllic existence is shattered by the entry of Rupali Walia (Sushmita Sen). Rupali, is an ambitious model, who has a single goal in life, to marry a rich man. As fate would have it Prem hires her to model for his agency and this is the start of an affair between the two. Rupali uses her feminine wiles to entrap Prem who tells her that he is unmarried. The lovers plan a getaway to Switzerland and their secret is discovered by Prem and Puja's friends, Lakhan (Anil Kapoor) and Lovely (Tabu). Lakhan, the family well wisher, pleads with Prem to break off from Rupali, but when Prem returns to India, he continues to meet Rupali. Unsuspecting Puja is in love with Prem, but one night she discovers Prem secretly visiting Rupali. Puja is schocked and issues and ultimatum to Prem to break off his liason with Rupali. Prem is unrepentant and leaves home. Puja is heartbroken and at her wit's end until Lakhan comes to the rescue. He advises to Puja to fight for her rights and Puja transforms from a simple homely girl into a woman who takes destiny into her own hands. She devises an amazing strategy to win back her husband from the clutches of the other woman. But will she succeed where others have failed?