C.C. "Bud" Baxter is a lonely office drudge at an insurance corporation in New York City. To climb the corporate ladder, he allows four company managers to take turns borrowing his Upper West Side apartment, 51 West 67th Street, for their extramarital affairs. Baxter meticulously juggles the "booking" schedule, but the steady stream of women convinces his neighbors that he is a playboy.
Baxter solicits glowing performance reviews from the four managers and submits them to personnel director Jeff Sheldrake, who then promises to promote him – but Sheldrake also demands use of the apartment for his own affairs, beginning that night. As compensation for this short notice, he gives Baxter two theater tickets for that evening. Bud asks his secret crush, Fran Kubelik, an elevator operator in the office building, to join him. She agrees but first meets up with a "former fling", who turns out to be Sheldrake. When Sheldrake dissuades her from breaking up with him, promising to divorce his wife, they head to Baxter's apartment, while Baxter waits outside the theater.
During the company's raucous Christmas party several weeks later, Sheldrake's secretary, Miss Olsen, tells Fran that her boss has had affairs with other female employees, including herself. Fran confronts Sheldrake at Baxter's apartment, but he claims he loves her, then heads back to his suburban family after presenting her with a "Christmas present" of a $100 bill.
Realizing that Fran is the woman Sheldrake has been taking to his apartment, Baxter lets himself be picked up by a married lady at a local bar. When they arrive at his apartment, he discovers Fran, passed out on his bed from an suicidal overdose of sleeping pills. He ditches the woman from the bar and enlists Dr. Dreyfuss, a medical doctor living in the next-door apartment, to revive Fran. When Baxter makes Dreyfuss believe that he was the cause of the incident, Dreyfuss scolds him for philandering and advises him to "be a mensch."
The next day, Mrs. Dreyfuss brings breakfast over for Fran and pleads with her to make a lifestyle change and settle down. Mrs. Dreyfuss and Fran repeatedly point out the poor housekeeping in Baxter's bachelor pad, including the fact that he uses a tennis racket as a spaghetti strainer in the kitchen. Dr. Dreyfuss recommends that Fran spend two days recuperating in the apartment, during which time she grows close to Baxter, especially after he confesses to his own botched suicide attempt over unrequited feelings for his best friend's wife, who now sends him a fruitcake every Christmas. Fran says she has always suffered bad luck in her love life.
As Fran is sleeping in Baxter's room, one of the managers arrives for a tryst. Baxter persuades him and his companion to leave, but the manager recognizes Fran and informs his colleagues. The next day, confronted by Fran's brother-in-law, Karl Matuschka, who is looking for her, the jealous managers direct Karl to Baxter's apartment. Baxter and Fran are in the middle of eating meatballs and spaghetti, which Baxter strained using the tennis racket, when Karl rings the doorbell and demands that Fran return home to her sister, who is concerned about her. Baxter deflects the brother's-in-law anger over Fran's wayward behavior by once again assuming all responsibility. When Karl finds out about Fran's suicide attempt, he punches Baxter, knocking him to the floor. Fran reacts by kissing Baxter on the forehead for protecting her, exclaiming softly, "Oh you fool, you damn fool."
When Sheldrake learns that Miss Olsen tipped off Fran about his affairs, he fires her, but she retaliates by spilling all to Sheldrake's wife, who promptly throws her husband out. With no regrets for his behavior, Sheldrake believes that this situation just makes it easier to pursue Fran, although she hints that she is losing interest. Having promoted Baxter to an even higher position, which includes a key to the executive washroom, Sheldrake expects Baxter to loan out his apartment yet again for him and Fran. Baxter refuses to let Sheldrake bring anyone to his apartment again, "especially not Miss Kubelik". When Sheldrake threatens to fire him, Baxter gives back the washroom key instead of the apartment key – proclaiming that he has decided to become a mensch – and quits the firm. He decides to move out of the apartment and begins to pack his belongings, including the tennis racket and the handgun from his suicide attempt.
That night at a New Year's Eve party, Sheldrake indignantly tells Fran about Baxter quitting rather than letting him bring her to his apartment. Realizing that Baxter really loves her, Fran abandons Sheldrake and runs to the apartment. Running up the stairs, she hears an apparent gunshot. Fearing that Baxter has shot himself, she frantically pounds on the door. Baxter opens the door with a bottle of champagne in his hand, having just popped the cork. As they sit down to finish a game of cards started during Fran's recuperation, she reveals that she is on her own, like he is. When he asks about Sheldrake, she replies, "We'll send him a fruitcake every Christmas,” prompting him to declare his love for her. She hands him the cards and affectionately tells him to "Shut up and deal".
