Irreversible

Alex and Marcus, a young couple, wake up together and discuss their relationship, as well as an upcoming party, while in the nude. Marcus voices concern about Alex's ex, Pierre, also being invited because he "stole his girl", to which Alex responds that he did not steal anything, because she is not an object and it was all her decision. Alex reveals she had a dream of herself standing in a red tunnel that breaks in two. The two talk about a potential pregnancy and Marcus expresses hope in one before Alex takes a pregnancy test, the result of which is never shown, but she seems particularly pleased with.

On a public train, Pierre, Alex's ex who is also invited to the party, constantly bickers with Alex over his inability to satisfy her during their relationship, while Marcus shows no interest in their squabble. Alex explains to Pierre that he always focused too much on her, and a sexual partner can feel and is aroused by their partner's pleasure. At the party, Marcus gets drunk and takes cocaine, much to Alex's disapproval. She leaves the party, asking Pierre to look after Marcus. Alex descends into a red pedestrian underpass on her way back to the train when she notices a transgender prostitute getting attacked by a man. He immediately turns his attention to Alex, anally raping her before savagely beating her into unconsciousness.

After Marcus and Pierre discover Alex being taken away by paramedics, they encounter street criminals Mourad and Layde, who offer to help them find the culprit. They use an ID left at the scene by the prostitute to locate her. Marcus verbally assaults Concha, the prostitute, and threatens to cut her face open in order to gather that the rapist's name is Le Ténia ("the tapeworm") and that he frequents a gay BDSM club called The Rectum. The men are chased off by other prostitutes; Marcus and Pierre jump into a taxi cab and speed off into the night. When the cab driver doesn't know where The Rectum is, Marcus attacks the driver, stealing his vehicle. The two end up finding the club's location, with Pierre reluctantly following behind Marcus. Leading the charge, Marcus proceeds to get into a fight with a man he suspects of being Le Ténia, who overpowers him and breaks his arm before attempting to rape him. Pierre comes to his rescue and beats the man to death with a nearby fire extinguisher as the man's companion, the actual rapist, watches in amusement. Marcus is carried out of Rectum on a stretcher while Pierre is arrested by police as Mourad and Layde deride them.

Meanwhile, in a nearby small apartment, a man named the Butcher tells a friend that he was arrested for raping his daughter before dismissing the commotion going on outside.