Chuck Baxter, an ambitious bachelor and junior executive for a large insurance company called Consolidated Life, expresses his frustrations and hopes for career advancement ("Half as Big as Life"). To curry favor with higher-ups in the company, he allows his apartment to be used for their romantic trysts in return for promises of promotion ("Upstairs"). Chuck has his own eye set on Fran Kubelik, a waitress in the company cafeteria whom he's always admired from a distance. While talking to Chuck, she wonders if she will ever find someone to share her life with. Chuck hopes that she might notice him ("You'll Think of Someone").
J.D. Sheldrake, the company's powerful personnel director, notices the glowing reviews written by Chuck's superiors and deduces the reason for them. He requests sole use of the apartment for his affairs in exchange for Chuck's long-awaited promotion and tickets to a basketball game ("Our Little Secret"). Chuck asks Fran to attend the basketball game with him, and she agrees to meet him there after first having a drink with her soon-to-be ex-lover ("She Likes Basketball").
Fran's lover turns out to be Sheldrake, who is married. Fran wants to end the relationship, but Sheldrake talks her into spending the evening with him ("Knowing When to Leave"). Though Fran stands him up, Chuck forgives her. When he informs the other executives that his apartment is no longer available for their use, they express dismay ("Where Can You Take a Girl?"). Meanwhile, Sheldrake wonders why he is drawn to affairs ("Wanting Things"). The scene shifts to the raucous company Christmas party ("Turkey Lurkey Time"). At the party, Miss Olsen, Sheldrake's secretary, reveals to Fran that she is simply the latest in a long line of Sheldrake's mistresses. The first-act curtain falls as Chuck discovers that Fran is the one Sheldrake has been taking to his apartment.
A despondent Chuck spends Christmas Eve trying to drink away his troubles at a bar. He meets another tipsy lonelyheart, Marge MacDougall, who agrees to come back to his apartment ("A Fact Can Be a Beautiful Thing"). In the meantime, at Chuck's apartment, Fran confronts Sheldrake about his earlier affairs. While he admits to the affairs, he declares his love for Fran, but tells her that he must leave in order to catch his train home to spend Christmas Eve with his family. A despairing Fran discovers Chuck's sleeping pills and takes the whole bottle ("Whoever You Are").
When Chuck arrives with Marge, he discovers Fran on his bed. After quickly disposing of Marge, a frantic Chuck gets his neighbor, Dr. Dreyfuss, to come over, and together they save her life. The next morning Chuck calls Sheldrake to let him know what happened. Sheldrake says he can't leave for the city without his wife knowing and asks Chuck to take care of Fran ("Christmas Day").
Over the next few days Chuck and Dreyfuss try to keep Fran's spirits up to prevent a relapse into suicidal behavior ("A Young Pretty Girl Like You"). Chuck and Fran play gin rummy and discuss their problems, growing closer ("I'll Never Fall In Love Again"). Mr. Kirkeby, one of Chuck's former 'clients,' discovers that Fran has been staying at Chuck's apartment, so as revenge for cutting him and the others off from using the apartment he tells Fran's overly protective brother where she has been staying. Karl Kubelik then comes to the apartment to collect her and, believing that Chuck is the cause of her current state, punches Chuck.
Miss Olsen soon discovers that Sheldrake's actions led to Fran almost killing herself. She quits her job and tells Mrs. Sheldrake all about her husband's affairs. She leaves him, resulting in his desperation to woo Fran back. Sheldrake asks for the keys to Chuck's apartment again on New Year's Eve to take Fran there. Chuck refuses and quits his job rather than allow Sheldrake to take Fran to his apartment ever again ("Promises, Promises").
Deciding that he has to get away, Chuck begins packing to move elsewhere when Fran comes to see him. Sheldrake had told her that Chuck had refused him access and quit, and she realizes that Chuck is the one who really loves her. As they resume their earlier game of gin, he declares his love for her, to which she replies, "Shut up and play cards."
J. D. Sheldrake
Miss Della Hoya
Miss Kreplinski (Nurse)
Employees of Consolidated Life, party-goers, restaurant patrons