The mentally ill Dr. Fenwick is a supporter of the therapeutic value of role-playing in psychiatry. He kills the head of the sanitarium where he is a patient and assumes the man's identity. He locks doctors and nurses in the attic and gives their responsibilities to other patients. Things run rather smoothly until Natalie Rivers, the niece of the now dead head of the sanitarium, arrives for a visit. Since she has never seen her uncle before Fenwick is able to trick her into thinking that he is her uncle. Natalie discovers her real uncle's body and, not knowing who he is, asks Fenwick to call the police. He pretends to call the police and a patient pretending to be a detective shows up to look things over. Eventually Natalie discovers the staff locked in the attic. Fenwick tries to prevent her from telling anyone, but he is stopped by the police. It seems that the patient playing the detective became so immersed in the role that he thought it was his professional duty to inform the authorities.