Zack Elliot (Michael Ontkean) is a successful young doctor in the Los Angeles area married to Claire (Kate Jackson), an equally successful television network producer. They have been married for eight years and are generally happy in their relationship, sharing in common a love for Gilbert and Sullivan and the poetry of Rupert Brooke, to whom they were introduced by their elderly former neighbor, Winnie Bates (Wendy Hiller). Intending to start a family, the couple buy a big house together.
Unknown to Claire, Zack has been struggling with feelings of attraction to other men. He picks up men in his car and starts frequenting gay bars in West Hollywood on his lunch hour, although he does not follow through sexually. This changes when he meets Bart McGuire (Harry Hamlin), a gay novelist who comes to see him for a medical check-up. Bart leads a fairly hedonistic single lifestyle, picking up multiple sexual partners, frequenting gay bars and clubs, occasionally taking recreational drugs. Zack and Bart are mutually but unspokenly attracted to each other and go out for lunch.
A few days later, Zack calls Bart and asks him on a dinner date. He lies to Claire, saying he has to work late. At Bart's house, Zack and Bart talk and it becomes clear that Zack is not yet able to identify as gay, instead labeling himself "curious". That night Zack and Bart go to bed together, which is the first time Zack has ever had sex with another man. Afterwards, Zack wants to stay the night, but Bart, following his usual pattern, brushes him off. Angered, Zack leaves but returns the next day to confront Bart further about Bart's fear of intimacy. Bart makes plans for them to get together during the weekend.
Claire, concerned about the growing distance in her marriage, goes to her boss seeking a year-long leave of absence. Instead, he promotes her and sends her to New York City on a weekend business trip. Zack takes advantage of the opportunity to spend more time with Bart, but they end up arguing. Zack calls the outline for Bart's new novel less than honest, and Bart confronts Zack about his own lack of honesty about his sexuality. That night in bed, Zack tells Bart that he loves him. The next morning, fearful of his own growing feelings for Zack, Bart pushes him away again.
(Throughout the film, Bart and Claire deliver several mini-monologues, speaking directly to the camera about aspects of their lives and their feelings about the scenes that had just played out on-screen.)
Eventually, Bart realizes that he does have feelings for Zack but that he is not ready for the level of commitment that Zack needs. He is last seen in the film back out in the bars, cruising.
With Claire home from her trip, Zack tells Claire of his feelings for other men. Although she said she could handle anything he could tell her, she reacts very badly and Zack leaves the house. A few days later, an emotional Claire trashes some of Zack's clothes and finds a matchbook with a man's name and number written in it. She locates someone Zack had picked up, and they talk. She learns from him that gay people can live normal and happy lives. Claire attempts to get Zack to remain in the marriage, even claiming that she would be okay with him having affairs with other men, but Zack advises her that she must let go. Zack then tells Claire that he has a job prospect in New York City, working with cancer patients. In the end, the two agree to a divorce.
The film jumps forward a few years later, with the death of Winnie Bates, Zack and Claire's former neighbor. Zack is living in New York and in a committed relationship with another man by this time, but he returns to Los Angeles for the funeral. Claire has since gotten re-married to an architect and has a young son named Rupert. After the funeral, Zack and Claire discuss their lives and express their own happiness and their gratitude that the other is happy.