Kirk and ship's psychiatrist Dr. Noel visit a Federation mental hospital as the Enterprise delivers supplies. But one cargo container beamed aboard the ship contains an apparently insane stowaway from the facility on the planet who isn't a patient, but the second in command of the hospital's director, who has invented a device that can lock emotional impulses in or out of the brain permanently and is apparently used his invention without any discretion. Spock and the crew discover that Kirk and Dr. Noel are trapped on the planet, and are probably the next victims of the mind-altering machine.
