Set in 1959 during the Algerian War, the film follows Lieutenant Terrien, Benoît Magimel, an inexperienced and naïve junior French Army officer who volunteers for active service rather than a safe staff post in Algiers. He is posted to Kabylie, a remote and mountainous region of Algeria, as a replacement for Lieutenant Constantin who was killed during a ‘friendly fire’ incident commanding a counter-insurgency ambush operation. The war is far more complicated than he anticipated as he takes over command of his new platoon at the outpost Mazel.
Within hours of taking over, he is ordered to lead a locate and destroy mission into the zone interdite (the Forbidden Zone) to find a World War II French Army veteran named Slimane, now a local commander of Algerian rebels seeking independence. Slimane is never seen in person during the film.
When the Fellagha massacre the population of a local village after a patrol visit, Terrien vows to stay calm and professional despite the appalling horrors that greet him. Terrien saves a young boy from drowning in the village well and is gradually forced to see the conflict through the eyes of that child: a child who temporarily adopts the French soldiers almost as a surrogate family. Terrien’s determination to remain detached, professional and controlled despite the atrocities around him, including the torture, abuse and summary executions of Algerian prisoners, slowly gains him the initial contempt of his men and begins to fracture his own resolve.
The blooding of the young Lieutenant and the way in which he reacts to his newfound knowledge and experience provides the dramatic arc of L’Ennemi intime. Terrien’s idealistic view of French involvement in Algeria is summed up during a conversation with Captain Berthaut, an intelligence officer, when Terrien argues that as metropolitan French citizens the Algerians should enjoy the same political rights as any other French citizen and states that "You can’t fight barbarism with barbarism." Marc Barbé plays Berthaut, a former member of the French Resistance who was tortured by the Gestapo, a veteran of the French Indochina War and an old comrade-in-arms of Dougnac. Berthaut counters that, "At 110 volts the truth always comes out." Berthaut is killed and mutilated during a Fellaghist ambush as he attempts to evacuate a wounded soldier.
Later, French soldiers retaliate by massacring and burning an entire village. In response, Terrien slowly begins to change his view about the war’s methods and the cycle of violence.
Dougnac, identified as a veteran of French Indochina, is a complicated character who is not averse to resorting to torture and barbarism when dealing with insurgents yet is capable, professional and increasingly privately respectful of Terrien as an officer and as a man. Albert Dupontel plays Dougnac. Their professional differences and the harsh realities of operations drive the two men to breaking points, with Dougnac finding release through alcohol and at least one instance of self-inflicted torture. Such pressures contribute to Dougnac deserting the army, while Terrien’s arc culminates in tragedy as he is shot by the boy he saved, whose brother was killed in a French military operation.