Myra Savage is a professional medium who holds séances for clients every Wednesday. She is assisted by her meek husband Bill, who is unable to work due to his asthma. She decides that she needs more publicity for her work. Bill has reluctantly agreed to help her stage a kidnapping so that she can gain public exposure by providing the police with the location of both the child and the ransom money. They re-decorate the old upstairs room of their dead son Arthur so that it will look like a hospital, blocking the window so that neither light nor sound can escape.
Bill leaves his motorbike at an isolated spot and then goes to a local school and waits for Amanda Clayton to be collected by the family chauffeur. Once she is in the car, he distracts the driver and drives her away. Going back to where he left his motorbike, he drugs Amanda and places her in the sidecar, then takes her to his house and places her in the room upstairs. Dressed in medical uniforms and wearing surgical masks, Bill and Myra check on Amanda, telling her when she wakes that she is in hospital with a special form of German measles. Bill and Myra then send a ransom note asking for a £ 25,000.
Myra goes to the Claymore family home offering her services as a medium. Mr Claymore is sceptical, but his wife is more receptive. Myra leaves her calling card and gives another to the police. That evening the police visit Bill and Myra, who pretend to be out. Bill is clearly upset by the police's sudden appearance, but Myra convinces him to go through with the plan. He goes into central London and calls from a phone booth to arrange the ransom drop.
The next day, Amanda is drugged again, placed in the sidecar and driven to a secluded area. Bill goes to collect the money, while the bedroom is emptied in case the police come again. Bill phones Mr Claymore, who is under police observation, and tells him to take the money to Piccadilly Circus underground station. Once the ransom is in position Bill waits until a large group of passengers head towards Claymore. He then grabs the bag and gets on to a train, eluding the police in the confusion. At the same time, a police sergeant is interviewing Myra while she shows him around the house. After he leaves, Bill brings Amanda back. She now has a fever and although Bill is seriously worried about her health, Myra won't budge from the plan.
The next day Myra holds her usual Wednesday séance, but this time it is joined by Mrs Claymore. While Myra is in a trance, Amanda starts to call out asking for her mother. Myra faints and Bill comes to her aid. After everyone has left, Myra tells Bill that their son Arthur wants Amanda to stay with him. Bill realises that she means to kill Amanda and angrily tells her that she is imagining it all - their son Arthur can't be speaking to her, as he was stillborn and Myra was never even allowed to see the boy. Awakened by the noise of the argument, Amanda appears at the top of stairs. Bill takes her back to her room, realising that he must have forgotten to lock the door when Myra fainted. Myra says that now they must kill her as she has seen the inside of their house and both of them without their masks. Bill drops the inert body of Amanda at the foot of a tree at a nearby forest and is seen leaving by a scoutmaster camping nearby.
The police come to the house and convince Myra to have a séance. While in a trance, she admits to having taken the child. Bill tells the police where he put the ransom money. They tell him that Amanda is recovering from her fever. Unaware of what is transpiring, Myra asks Bill if the séance went well.
