In spite of the obviously psychedelic packaging for this film, The Mind Benders is marginally science fiction. Instead, this is a drama with SF trappings. The story of a scientist, Dr. Longman, played by Dirk Bogarde, who comes to believe that the suicide of a colleague was caused by prolonged exposure in a sensory deprivation tank, he is forced to prove his theory by a British intelligence agent and undergo the same treatment himself.
The agent is convinced that Longman's colleague was a spy and engages in some subtle brainwashing of Longman after he's spent eight hours in the tank. While the opening half hour is definitely gripping--including Longman's harrowing experience in the tank--the final two thirds of the film is sorely disappointing for science fiction fans--or even for those who are expecting a substantial buildup focused on the theme.
The development after the first 30 minutes or so is pretty weak, centering on Longman's relationship with his wife which has been put to the test, and is a real letdown. Although Dirk Bogarde and Mary Ure as his wife are more than competent actors, the film runs out of steam long before its happy ending.
The real value of this film, as indicated in the liner notes of the DVD, is that it is a precursor to Altered States and The Manchurian Candidate. For completists who want whatever film they can find about brainwashing, this is a must. For anyone else, it's really not great. A far better vehicle for Dirk Bogarde fans, also available on DVD, is The Servant.