At six in the morning, a cleaning lady unlocks the door of the locked ward of a psychiatric hospital and finds a bloodied corpse of an orderly who worked the night shift. She informs the doctor on duty, who determines the violent death by a blow to the head and informs the police. The case is assigned to the detective Richard Teller. The crime scene reminds him of a sealed container such as an aquarium; at the time of the murder, the ward was locked, with only an orderly and fifteen patients, so they become logical suspects. The detective delves into their diagnoses and talks to each of them personally, but this brings him more questions than answers. The main question is: can a reasonable motive for murder be expected from people whose mind is not reasonable because of illness?