Felix Sherman (Segal), a nebbish book clerk and aspiring novelist, struggles to maintain peace and quiet in his walk-up New York City apartment. When he reports to his landlord that his brass, uneducated neighbor Doris (Streisand) is working as a prostitute, she is suddenly evicted. She then confronts him about this immediately, in the middle of the night. Felix, who hadn’t intended that she actually be evicted, reluctantly agrees to let her stay at his apartment on a temporary basis.
However, soon after on that same night, Doris accidentally sees Felix naked, causing her to laugh and to develop a furious case of the hiccups. She asks Felix to scare her so that she stop hiccuping. He obliges, dressing up in a skeleton Halloween costume and jumping at her suddenly. She reflexively acts in self-defense and the noise of their scuffle causes the landlord to barge in and evict Felix.
Now both evicted, they relocate to the upscale apartment of Felix’s friend and coworker Barney (Klein). The two very different personalities continue to clash throughout the night as Felix tries in vain to maintain his routine and to finally get some sleep. The TV-loving Doris gets upset after Felix reads her an excerpt from his novel, which she vehemently dislikes, and the noise from their argument causes Barney to leave. Nevertheless, as Felix and Doris get to know each other better, discussing topics like Doris’ various stage names, they grow to like each other and they make love. In the morning, however, their fighting resumes and Doris leaves in anger.
Felix and Doris go back to their lives and both struggle professionally. Felix walks past a theater showing an adult film starring Doris and, though out of place in the sordid atmosphere, he decides to watch it out of curiosity. He gets uncomfortable and leaves midway through the film. Now a week after Doris had left, one of Doris’ friends Eleanor (Kelly) goes to the bookstore where Felix works and mistakenly confronts Barney instead of him. Eleanor then tells Felix that Doris is at a cafe, where Felix goes to meet her. They two walk around the city near Lincoln Center, now clearly drawn to each other and Felix is impressed by how Doris has been trying hard to expand her vocabulary. However, their night is interrupted when they have to run away from a group of violent youths.
They then go to the fancy townhouse where Felix is staying, where Felix discovers more details about Doris’ past that make him uncomfortable and where Doris discovers that Felix is actually engaged to be married. Doris puts Felix in bed and tells him both that she is planning to move to Los Angeles and that she has thought about the excerpt from his novel that he had read to her before and now really likes it. He kisses her and they make love again. They then get stoned and continue to bond. Suddenly, however, Felix’s financee (Anson) and her parents (Manning, Carney) return and discover a high Felix and Doris with each other in the bathtub.
Kicked out of the townhouse and no longer high, Doris and a frustrated Felix, carrying his typewriter, walk together in Central Park that morning. They start to argue and Felix cruelly makes fun of Doris on top of Cedar Hill. She starts to cry and a regretful Felix kisses her hand before she smacks him. Felix admits to her that his actual name is Fred and he tosses his typewriter down the hill. Doris admits that her full name is Doris Wilgus. Now, finally without any pretensions between each other, they walk away together, as a happy couple.