The central twist of "The Apartment" is the revelation that Fran Kubelik, the elevator operator Bud Baxter is infatuated with, is the same woman his boss, J.D. Sheldrake, has been bringing to his apartment. Bud discovers this on Christmas Eve when Sheldrake gives him tickets to a show, and Bud sees Fran's broken compact mirror, which he recognizes from their earlier conversation. This reveal shifts the film's tone from light cynical comedy to poignant drama, as Bud is now a direct, unwilling accomplice in the heartbreak of the woman he loves.
The film's dramatic midpoint occurs after Fran, distraught upon learning from Sheldrake's secretary that she is just one in a long line of his mistresses, attempts suicide in Bud's apartment by overdosing on his sleeping pills. Bud returns home with another woman to find Fran unconscious in his bed. He and his neighbor, Dr. Dreyfuss, manage to revive her. This event is the catalyst for Bud's moral transformation. In nursing Fran back to health over the next two days, they form a genuine bond, sharing stories of past heartbreaks, including Bud's own suicide attempt. He falls completely in love with her.
The climax occurs on New Year's Eve. Sheldrake, having been kicked out by his wife, believes he can now have Fran all to himself and demands the apartment key from Bud. In a defining moment of courage, Bud refuses, holding up the key and declaring, "The old hygiene is back." He quits his high-paying job rather than continue to enable Sheldrake's exploitation of Fran. Meanwhile, at a New Year's party with Sheldrake, Fran learns of Bud's sacrifice. Realizing who truly loves her, she abandons the stunned Sheldrake mid-sentence and runs to Bud's apartment. She arrives just as he is packing to leave, and the film ends with her dealing a deck of cards and delivering the iconic line, "Shut up and deal," signifying the hopeful, unwritten beginning of their life together.