There is an attempted assault in the home of an old woman, who defends herself by shooting at the robbers, though she is not just an old lady, but the mother of a serial killer, in prison for killing four women, and the blood trail left by one of the attackers shares 50% of the DNA with Lieutenant Lebellec. Erwan must reveal that ten years ago, when he was a student, his best friend, Johanna Dupieux, suddenly disappeared and Erwan's mother tries to demonstrate that Johanna was the victim of a different assassin.
—mgallegog