Max [Max] is a former bohemian and an amateur writer who takes a job in New York, leaving his girlfriend Lisa Lisa, with whom he was madly in love, in mysterious circumstances. After two years, he returns to Paris, decides to settle down, and becomes engaged to Muriel. By chance, he catches a glimpse of Lisa in a café, but cannot reach her before she storms out. Determined to meet her again, Max secretly cancels a business trip abroad to pursue his lost love. Through a series of ruses and perseverance, he enters Lisa's apartment and, hearing that somebody else has arrived, hides in her wardrobe.
First, he thinks it is Lisa, as the girl who came to the apartment resembles Lisa from behind. After several misunderstandings, they finally get acquainted. The girl introduces herself as Lisa. The same night, they make love, and their relationship starts to develop. The girl's real name is Alice. During the film, flashbacks are intertwined with the narrative to provide a background for Max, Lisa, and especially for Alice, shedding light on the situation.
The flashbacks reveal that Alice and Lisa were best friends living in apartments on the same floor of two facing buildings, and that Alice became obsessed with Max, Lisa's then-boyfriend, from a distance. She restyled herself to look like Lisa while secretly engineering a breakup between them. Lisa is a stage actress and leaves abruptly for a two-month tour, giving Alice a letter to deliver to Max; Alice never sends the letter. Max, believing Lisa left because she didn't love him, accepts a job in New York and leaves. Upon her return, Lisa is heartbroken that Max has left her and leaves on a cruise (a gift from Alice) to ease her mind, where she meets a rich, married older man named Daniel.
Lisa is being pursued by Daniel, who might have murdered his wife to get closer to her. For this reason, she avoids her flat and lets Alice use it. To complicate matters further, Alice is dating Max's best friend, Lucien.
In a sub-plot, Alice is seen acting in Midsummer Night's Dream, drawing comparisons between the four lovers in the film and those in the play; it is arguable that the whole film is a rendition of the play.
The film begins and ends in reality, where Max and Muriel lead a futile, vacuous but bourgeois life in high finance (and the implication being that Muriel is the boss's daughter, thus imitating Egeus' involvement in Hermia's marriage to Demetrius in Midsummer Night's Dream), but almost all the action takes place in a dreamlike trance where the lovers don't really know whom they love. Lucien remains faithful to Alice, and pursues her, but both Alice and Lisa (names that imply reflections of each other) initially both love Max, and Max, although madly in love with Lisa, turns to Alice after reading her diary, just before reality dawns and he accepts his fate with Muriel.
Lisa returns to her apartment and is confronted by Daniel, who drops a lighter on the floor (covered in gasoline) causing the apartment to explode and blowing Lucien through the window of a café across the street. Alice leaves her life behind and flies to Rome, bidding Max one last farewell glance as he embraces Muriel at the airport.
