Greg, Vicky, and Justine arrive in town and are looking for an apartment near college. Each of them wants to rent the 5A for themselves alone, but the rent is too expensive: they have no choice but to become roommates. On the first night, Justine discovers, behind the wallpaper in her room, a hidden door. The roommates uncover this door, which opens onto a closet lined with copper, in the middle of which lies a mysterious trunk containing the skeleton of a young man. By opening the trunk, they accidentally release the ghost of Théophile Langevin, who has been trapped there since 1957 in a ghostly coma.

The disappearance of Gonzague prompts the Ghost Society to send Schmitt to the apartment to investigate the events. Following his recommendations, Théo is assigned a new ghost-master: Éléonore des Prés, who proves to be a strict and cruel woman with him.

Back in the present after their 1957 trip, the roommates realize that Justine has disappeared! They find out she was kidnapped by UltraCorp and do everything they can to find her. Meanwhile, the Ghost Society has decided to sue Théo for the destruction of Éléonore Després.