Paul Prieur (Cluzet) runs a lakeside resort. He is married to Nelly (Béart), with whom he has a son. Paul starts suspecting that Nelly may be having an affair, and despite having no concrete proof and Nelly's assurances that it's not the case, he obsesses over the possibility and grows increasingly delusional, paranoid and violent. Life for the married couple turns into a living hell as a result.
Coincidentally, there's another French film called L'Enfer made in 2005 that also stars Béart. The two films are unrelated.
- Alone with the Psycho: In the end, Nelly is alone in her bedroom with Paul. She realizes that he is completely crazy and dangerous for her. Moreover, she notices that Paul has locked the door.
- Ambiguous Ending: Given Paul's delusional state, whether he murders Nelly or she's still sound asleep in bed at the end is very much up in the air.
- Ax-Crazy: Paul becomes totally crazy and violent with Nelly by the end of the film. Whether he really murders Nelly or not is ambiguous.
- Chekhov's Gun: Paul's razor. Early in the film, Paul is seen shaving with a straight razor. In the end, he uses it to cut Nelly's throat (whether this is real or not is ambiguous, because this scene might be a Fantasy Sequence).
- Crazy Jealous Guy: Paul grows insanely possessive of Nelly with no concrete cause for it. Nelly thinks it's cute at first, but as Paul becomes more unhinged she really starts wishing she could just get the hell out of there.
- Destructive Romance: Paul and Nelly truly love each other. At first, Nelly tolerates Paul's abuses because she loves him and cannot live without him. Paul eventually go mad and Nelly is violently abused and maybe even murdered by Paul.
- Domestic Abuse: Besides the psychological abuse Paul heaps on Nelly, constantly accusing her of lying all the time, he also forbids her from leaving their home and eventually rapes her.
- Fantasy Sequence: There are many sequences that show how Paul imagines his wife's unfaithfulness.
- Green-Eyed Monster: Paul is jealous. He imagines that his wife, Nelly, cheats on him. This makes him crazier and crazier and in the end he is a real danger for Nelly's life.
- Hearing Voices: Paul has an inner voice that poisons his thoughts against Nelly and feeds into his paranoia. Occasionally he has conversations with the voice. Clearly he's more than a little mentally imbalanced.
- Imaginary Love Triangle: Paul imagines that his wife, Nelly, cheats on him with Martineau. Whether Nelly is really attracted to Martineau is left ambiguous at first, but in the end it is quite clear that she only loves Paul.
- Improperly Paranoid: There's zero evidence that Nelly is having an affair; the most Paul sees is her just hanging out with friends. But he's absolutely convinced that she must be going behind his back.
- Love Makes You Crazy: Paul falls in love with Nelly, but he is so jealous that he becomes crazier and crazier, so much so that he finally threatens Nelly's life.
- Mistaken for Cheating: Happens several times, because Paul always thinks that Nelly cheats on him. For example, Paul sees Nelly walking in front of Martineau's garage. She turns the corner and just thereafter Martineau goes out of his garage in his car. Paul imagines that he picks up his wife behind the corner, out of his sight. Later, he sees his wife water-skiing with Martineau. Then he sees her and Martineau docking on an island. He imagines that they make love on the island. Nelly's later action makes clear that she did not cheat on her husband with Martineau.
- Sanity Slippage: Because of his jealousy, Paul progressively becomes crazy.
- That Liar Lies: When he is angry at her, Paul tells his wife Nelly that she constantly lies.
- Through the Eyes of Madness: In the final scene, lines blur between reality and what Paul's deranged mind imagines. For example, it is unclear whether the murder of Nelly or the ambulance are real or not.