Two lovers, Florence Carala and Julien Tavernier, devise a plan to murder Simon Carala, Florence's husband and a wealthy French industrialist who also happens to be Julien's boss. On a Saturday night when the office is quiet, Julien, a former Foreign Legion parachutist with a history in Indochina and Algeria, climbs the exterior of the building to Simon's private office and carries out the deed with his own gun. He then stages the scene as a suicide and exits with a secretary and a security guard who are meant to supply him with airtight alibis. In a cruel twist of fate, he glances up from his car and realizes he left his climbing rope hanging from the balcony rail. With the engine idling, he rushes back inside and boards the elevator, just as the power is cut and the weekend locks are engaged, trapping Julien between floors.
Meanwhile, Louis, a young crook with limited means, and his girlfriend Véronique, a flower shop assistant, spot Florence at a nearby café as she waits for Julien. The sight of Véronique makes Florence fear Julien has abandoned their plan for another woman, and Florence begins wandering the night, her hopes tangled with fear and longing as she searches for him in bars and clubs around the city.
Louis and Véronique end up driving Julien's running car, and their path intersects with a pair of Germans in a sporty Mercedes. The jovial Horst Bencker invites them to share a drink with him and his wife Frieda Bencker at a motel. The two couples pass a few quiet hours in dialogue and curiosity, while Frieda snaps pictures of Louis and her husband with Julien's camera. When the film roll is finished, Véronique drops the developed roll at the motel's photo lab for processing.
After the Benckers retire, Louis tries to steal the Mercedes but is thwarted; a furious Horst is shot to death by Louis, and Frieda is killed in the same incident. Louis and Véronique drive back to Paris, where Véronique persuades Louis to join her in a plan to commit suicide with phenobarbital. The police soon connect the dots: Julien's car, gun, and raincoat are found near the Benckers’ bodies, and Simon Carala’s corpse is discovered at the office, while the building’s power is restored and Julien is inadvertently freed from the elevator, only to be arrested later at a cafe. He is charged with the murder of the Benckers, while Florence remains a person of interest in his disappearance and the staged crime.
Florence, determined to make sense of the tangled night, tracks down Véronique’s address and discovers that Véronique and Louis are still alive. She accuses the pair of murdering the Benckers and calls the police with an anonymous tip. Louis, who catches a glimpse of Julien’s mug shot in the papers, fears he may have underestimated how closely the police are watching. He races to the motel’s lab to destroy evidence, but his pursuit is cut short when he is arrested at the lab. Inside the lab, Commissaire Cherrier reveals the current evidence: photographs of Florence and Julien that were developed from Frieda’s camera, linking the lovers to the crime. The officers inform Florence that she will likely face a harsher sentence than Julien, and she answers with quiet resolve: she did it for love, and she believes she will someday be reunited with Julien.
