When Prof. Dirkbacher returns to the gymnasium at the end of the parent-teacher conference, he discovers the body of his colleague Prof. Ringelstein in the cloakroom. Someone had shot her in the head. Before he can notify the police, he hears footsteps in the stairwell: the murderer is still in the building! As the officers begin their investigation, they soon find out that the victim was not liked by all her colleagues. But what surprises the inspectors even more is that at the time of the crime, not only the murderer and Prof. Dirkbacher were in the school building.