At a sawmill, two teenagers witness a mass murder. Afraid of revealing their relationship, Yann and Lucas remain silent. Hélène Siquelande, Yann’s stepmother, leads the investigation. The boys’ lie triggers a dangerous chain reaction as the killer sets out to hunt them down.

In Perpignan, an explosion in the city centre targets the nightclub owned by Milo Ubieta, the father of Lena, a runaway teenager. The apartment where she was being held is destroyed, allowing her to escape and go in search of “Lancelot.” Hélène Siquelande attempts to establish a link with the sawmill massacre, suspecting these events are not a coincidence.

Lena, a teenage girl, is found dead. Yann and Lucas are blackmailed with compromising photos of their secret encounters. The media seizes on the case, putting pressure on Hélène Siquelande, who struggles to solve the crimes with very little evidence.

Yann breaks his silence, causing family tensions and a breakup with Lucas. Hélène Siquelande suffers a professional humiliation when Ronan Berg takes over her investigation—unaware that Berg is actually “Lancelot,” the killer she is pursuing, who is threatening Yann and Lucas.

Hélène struggles to keep Yann at home as the investigation finally makes progress. Lucas, crushed by his lie, confesses to Hélène that he was the sole witness to the killings in the woods. He hands over the murder weapon, prompting Hélène to request judicial protection for this key witness.

The investigation takes on a national dimension, forcing Ronan Berg, a.k.a. “Lancelot,” to give a televised interview. Fearing recognition by the two teenagers, he attempts to murder Lucas. Hélène discovers that Yann is the second witness and decides to flee with her family to protect him.