An aspiring executive lets his bosses use his apartment for assignations, only to fall for the big chief's mistress.

In New York in November 1959, C. C. "Buddy" Baxter toils in anonymity in the vast, impersonal offices of Consolidated Life Insurance. At his small apartment, however, Buddy has attracted the attention of several Consolidated executives who "borrow" the space for their extramarital trysts. Buddy, continually assured that he will gain a speedy promotion in thanks for his extra apartment key, endures repeated indignities and spends many of his nights walking the streets, looking up longingly at his own window. In addition, the constant flow of women to the apartment earns Buddy the antipathy of his neighbors, including kindly Dr. Dreyfuss and his wife Mildred, who assume that he is a callous playboy. In reality, Buddy lives a quiet, lonely life, and one night when Consolidated manager Joe Dobisch insists on using the apartment, Buddy contracts a cold while sitting outside in the rain waiting to be allowed back in. In the morning, Buddy works up the nerve to talk to elevator girl Fran Kubelik, who has a reputation among the executives as being hard to get. After spending the afternoon juggling apartment "appointments" so he can rest alone that evening, Buddy is called to the office of personnel manager J. D. "Jeff" Sheldrake, who confronts him about his popularity with the various executives. Although Buddy is worried he will be fired, in reality the married Sheldrake is attempting to intimidate him into lending him the apartment key, and despite his cold, Buddy is buoyed by Sheldrake's promise of an executive position. Unaware that Sheldrake's current girl friend is Fran, Buddy asks her to a play that evening. Because Fran is planning to break up with Sheldrake, she tells Buddy she will join him after meeting her "date" briefly. Fran later meets Sheldrake at a bar and tells him it is too painful for her to date a married man, but he convinces her that he is just about to ask his wife for a divorce. While Buddy waits at the theater, Sheldrake takes Fran back to Buddy's apartment. Soon after, Buddy receives his promised promotion and proudly marches away from the 17th floor's endless rows of underlings into a private office on the 19th floor. Dobisch and the other executives, frustrated that they have not been allowed to use the apartment lately, threaten Buddy's new job but he remains securely in Sheldrake's good graces, still unaware that Sheldrake's constant dates at the apartment are with Fran. At Christmas, the 19th floor hosts a party at which most of the company's employees carouse and imbibe. Buddy is thrilled to see Fran but does not realize that Sheldrake's secretary, Miss Olsen, has just informed Fran that Sheldrake routinely seduces all the women in the office, using the same speech to make each conquest. Dazed, Fran barely listens to Buddy's conversation, and when she pulls out her compact, he recognizes it as the one Sheldrake's "girl friend" once left at the apartment. Upon learning that Sheldrake plans another tryst that evening, a distraught Buddy retreats to a nearby bar, becoming ill-humoredly drunk with melancholy stranger Margie MacDougall. Meanwhile, Fran meets Sheldrake at the apartment and, receiving his Christmas gift of a $100 bill, becomes despondent. After Sheldrake leaves, Fran swallows Buddy's bottle of sleeping pills and passes out on his bed. When Buddy returns with Margie, he finds Fran and, throwing Margie out, rushes to Dr. Dreyfuss to ask for help. Dreyfuss, assuming that Buddy has mistreated Fran and driven her to suicide, excoriates Buddy while ministering to Fran. Under his care, she survives, and they return her to bed. Although Dreyfuss wants to report the situation, Buddy talks him out of it, after which Dreyfuss urges him to be "a mensch ," the Yiddish word for a good human being. The next morning, as Sheldrake is celebrating Christmas with his family, Buddy calls to inform him of Fran's condition, and Fran awakens in time to hear Sheldrake refuse to talk to her. When she tries to leave, Buddy detains her, both for her safety and to keep her near him as long as possible. Mildred agrees to prepare breakfast for Fran, and delivers soup along with a lecture to Fran to forget Buddy and marry a nice boy. Buddy plays cards with Fran until she falls asleep, assuring her that this Christmas is vastly preferable to his typical lonely holidays. Soon, Consolidated executive Al Kirkeby arrives with his girl friend, Sylvia, but upon spotting Fran in the bed, congratulates Buddy and leaves. When Fran wakes and wonders who would mind if she died, Buddy confesses that he would mind very much, and Fran questions why she never falls in love with "nice guys like you." The next morning, Sheldrake fires Miss Olsen, who after eavesdropping on his brief phone conversation with Fran, arranges to meet Sheldrake's wife to inform her about her husband's infidelities. Back at the apartment, Buddy attempts to prepare a nice meal for Fran using a tennis racket as a spaghetti strainer. During a discussion of their romantic misfortunes, Buddy admits that he once bought a revolver and accidentally shot himself in the knee while contemplating suicide. Just as they are ready to eat, Fran's brother-in-law, Karl Matuschka, comes over, tipped off by the disgruntled Dobisch and Kirkeby. At the same time, Dreyfuss visits, and when he inadvertently reveals to Karl that Fran overdosed, Buddy takes the blame to save Fran's reputation, earning himself a black eye from Karl and a grateful kiss on the forehead from Fran. In the morning, he prepares to inform Sheldrake that he will "take Fran off his hands," but Sheldrake announces that his wife has kicked him out so he plans, after an interlude to enjoy his bachelorhood, to reunite with Fran. Buddy's depression is only slightly mollified by the news that he has been promoted to Sheldrake's assistant, with a 24th floor office and key to the executive washroom. On New Year's Eve, however, when Sheldrake asks for the apartment key to rendezvous with Fran, Buddy refuses and quits, informing Sheldrake that he has decided to become a mensch . That night at Buddy's apartment, while he packs his belongings, including the revolver, Fran attends a party with Sheldrake and learns that Buddy quit rather than allow him to take Fran to his apartment. Finally realizing that Buddy loves her more than Sheldrake does, she slips out of the party and races to Buddy's apartment. On the stairs, she hears a loud crack, and fearing that Buddy has shot himself, pounds on his door, only to discover that he has merely popped open a bottle of champagne. As Fran settles down to deal a game of cards, Buddy proclaims his love to her, and cheerfully, she tells him to "shut up and deal."