In a ballroom at the Plaza Hotel, the fussy, neat, and perfectionist Felix Unger takes charge of a dress rehearsal for his daughter Edna's wedding. The rehearsal drags on and clashes with the hotel’s schedule, and Felix’s sharp ultimatum over an unfulfilled request leads the assistant manager to revoke their reservation. Gloria Unger, Felix’s wife who would later remarry him, decides to kick him out of the house for two weeks so she can plan the wedding without his exacting interference.
Felix returns to the apartment of his former roommate, Oscar Madison. Oscar’s voice is raspy and hoarse after having a vocal cord removed due to throat cancer. (In real life, Klugman had throat cancer surgery in 1989.) Moved by Oscar’s condition, Felix arranges a friendly poker night with Oscar’s usual crew—Murray Greshler, Speed Deegan, and Roy—and asks them to always let Oscar win. Speed, true to form, cracks under the pressure, beats him, and spills the beans. Oscar forgives them and stays part of the group. To help Oscar regain some confidence, Felix suggests voice exercises that strengthen his vocal cords, opening the door to a new gig: a voice-over for a television commercial that features the voice of a toilet cleaned with the advertised product. Later, Felix arranges a return to work for Oscar with the New York Examiner, this time as a personal advice columnist. Oscar reluctantly accepts, but only until his former secretary, Myrna Turner, transfers over to help him; inspired by the change, he eventually resigns and passes the column to her.
Felix hosts the engagement dinner at Oscar’s apartment because Gloria can’t manage it at her place while painters are still at work. During the evening, Felix bonds with Edna’s fiancé, Bill Sutton, but soon learns that Bill has been married and divorced twice, a discovery that sours the mood. Felix goes to meet Bill’s first ex-wife, a formidable photographer, and Edna asks Oscar to tell Felix to ease up. Oscar tries, but Felix initially vows to stop. That resolution falls apart when Felix track downs Bill’s second wife—a nudist whom he’d once photographed with—who is revealed through a photo returned to Gloria as the source of the scandal. Gloria, furious, kicks him out of the wedding permanently and even files an injunction.
Undeterred, Oscar steps in to walk Edna down the aisle and deliver a heartfelt speech, while Felix hides behind shrubbery and then strides in to take his place. The ceremony proceeds with lively energy, and the reception that follows is equally animated. Edna cleverly tosses the bouquet to Jeannie before stepping into the limo, signaling a bright continuation of the newlyweds’ story. In the end, Oscar confesses that he’d be willing to accept the toilet-voiceover job, but Felix reveals that spot has already been filled; instead, he offers Oscar a position as the voice of the deep fryer in a potato chip commercial, which Oscar eagerly accepts.