Neurosurgeon Laurence Jeffries (Anthony Perkins) decides there is only one way to deal with his unfaithful wife Frances Jeffries (Jill Ireland), and he concocts a diabolical plan to set a potentially dangerous person upon her as punishment. A man wandering on the beach seems to be just the right person to suit the needs of Dr. Jeffries. Finding an amnesiac with no identity (Charles Bronson) whom Jeffries realizes his discovery has violence in his heart, the doctor brings him home for treatment. Jeffries knows the Stranger has killed before, and now he begins an elaborate plan to cause the in-house patient to kill Frances. He slowly convinces the amnesiac that Frances is the confused man’s cheating wife. Dr. Jeffries also decides to have his manipulated patient kill off his wife’s lover Paul Damien (Henri Garcin) as well.