Aliens steal Spock's brain and the crew must retrieve it, with McCoy temporarily guiding the body.
A mysterious woman beams aboard the Enterprise and steals Spock's brain, leaving his body alive but empty. Kirk and McCoy track down the thief to the planet Sigma Draconis VI, where a society of women uses the brain to control their underground world. McCoy must repair Spock with the aid of a strange device.
The episode opened the third season and is often named as one of the weakest, but it has a loyal fan following for its cheerful absurdity. Its unusual premise has been quoted in many parodies, and it shows McCoy performing an operation guided by a machine.