Postman Joe Higgins (Moore Marriott) has a minor operation for a fracture at Heron’s Park hospital and dies while being put under at the operating table. The new hospital head, Dr. White (Ronald Adam), is worried about an inquest and his reputation more than he is about the victim. At the hospital dance that night, an hysterical nurse, Sister Marion Bates (Judy Campbell), stops the gramophone and announces that the postman’s death was no accident but was murder and runs out of the building. Bates is soon found stabbed to death on the grounds while dressed in an operating gown. Eccentric Scotland Yard inspector Cockrill (Alastair Sim) is called in to investigate and announces that there were two murders and the suspects are the three nurses and two doctors who were in the operating theater with Higgins. The five are: the womanizing surgeon Dr. Eden (Leo Genn); the second time an inquest was held for him over the death of a patient, the anesthesiologist Dr. Barney Barnes (Trevor Howard); the one with a past secret whose voice frightened the postman before the procedure, nurse Woods (Megs Jenkins); the one on the verge of a nervous breakdown over her mother’s death, nurse Esther Sanson (Rosamund John); and the pretty one who that night broke her engagement with Barnes and was seen kissing Eden, nurse Freddie Linley (Sally Gray). The inspector warns that there are missing poison tablets from the medicine cabinet and the killer will strike again at one of them if feeling threatened and orders them to only confide vital info to him. The inspector is right and soon another one of the five finds their life in danger.