Professor Sharpey (Harold Goldblatt) commits suicide after subjecting himself to sensory deprivation experiments in a secret research laboratory. His former colleague Doctor Longman (Dirk Bogarde) volunteers to undergo the same tests to try and establish what drove Sharpey to kill himself. He hopes to clear Sharpey's name with the police, who favour the theory that Sharpey was a double agent, perhaps allied with Communism, and that his death was the final result of his shame over betraying Great Britain. Longman begins subjecting himself to sessions in a sensory deprivation tank in order to prove that use of the technique can make one unusually susceptible to brainwashing or hypnotic suggestion.