August 1944. A postman is injured following a flying bomb attack and is taken to the local hospital, a converted Elizabethan manor. While being operated on he mysteriously dies, with no apparent cause. The anaesthetist, Dr Barnes, was involved in a similar case four years previously and so the hospital administrator asks him to remove himself from operations until an investigation has been completed. Barnes, incensed, refuses and joins a party for the hospital staff.
At the party, Barnes talks with Sister Bates, who has drunk too much and is still angry with the surgeon, Mr Eden, over the end of their affair. She tells Barnes that Eden now seems to have his sights set on Nurse Linley, Barnes' fiancée. Bates interrupts the dance and hysterically announces to all those present that the postman was in fact murdered. She claims not only to know how and why this was done, but to be able to prove it. She leaves the party and goes to the operating theatres. While trying to retrieve some evidence, she is stabbed by a person wearing a mask and a surgical gown. When her body is later discovered, a lethal dose of pills is also found to be missing.
Inspector Cockrill arrives and immediately limits the suspects to those who operated on the postman and who were at the party: Mr Eden, Dr Barnes, and three nurses, Frederica Linley, Esther Sanson and Nurse Woods. The Inspector tries to provoke all the suspects into making some damaging admissions. He learns that Nurse Sanson is still recovering from the death of her mother, who was left for dead when their house was bombed. Eventually she was found still alive in the rubble, but was so weakened that she only managed to hold on for a few days longer.
Barnes tells Cockrill that the postman was unnerved when he heard Nurse Woods' voice shortly before being operated on. He later discovers that the postman had been listening to a broadcast of anti-British propaganda from Hamburg just before being injured. It eventually transpires that Nurse Woods' twin sister has been making those broadcasts.
During another interrogation, Barnes and Eden get into a fight over Nurse Linley. Inspector Cockrill warns all the suspects to share any clues that they have on the case only with the police. Barnes reminds Nurse Linley of this when she seems to have a piece of information to share with the other suspects. Later Barnes summons the Inspector to his office. Just as the Inspector arrives, they hear a cry for help from the nurse's lodgings. They find that the gas has been left on and that Nurse Linley has almost suffocated and has been knocked unconscious while Nurse Sanson tried to help her down the stairs.
The Inspector decides to set a trap for the murderer. He tells the suspects that Nurse Linley, who has in fact completely recovered, has suffered a cranial fracture in her fall and needs urgent surgery. He arranges for all the suspects to operate on Nurse Linley and so re-creates the scene of the first murder. When Nurse Linley seems to be on the point of suffering the same mysterious death as the postman, the Inspector realises that a cylinder of gas, normally painted green, has been repainted black to resemble an oxygen cylinder. He stops the operation and accuses Nurse Sanson of the crime. The postman had been part of the team of rescue workers who left her mother to die. She then killed Bates, who had found one of the surgical gowns stained with the incriminating black paint.
Nurse Sanson flees the scene and Eden tries to inject her with a syringe. The Inspector stops him, but too late discovers that Nurse Sanson had swallowed the missing pills and that Eden was only trying to administer the antidote. Nurse Sanson dies and Cockrill leaves, offering his resignation to his superiors.
