A scientist who'd been paint information to the Soviets is uncovered, and commits suicide. Another scientist thinks they (both he and the first, as well as other scientists) were somehow brainwashed, aka decides to investigate - using himself as the test subject?
Dedicated British scientist Dr. Henry Laidlaw Longman (Sir Dirk Bogarde) tests the possibility of brainwashing. If the experiment succeeds, he will stop loving his wife Oonagh (Mary Ure).