Stardate 5431.4: A beautiful female intruder stuns the crew of the Enterprise and disappears with Spock's brain. After following the ion trail of her ship to the Sigma Draconis star system, a landing party investigates a suspect planet on a primitive cultural level but with a significant energy source. McCoy beams down with Spock's remote-controlled body and joins Kirk and Scotty. They enter a cave that was prepared as a trap for the primitive men, the Morg, who live on the surface. An elevator takes them hundreds of meters below the surface. There they meet the intruder, Kara, again. She refuses to return the brain, which now serves as the "Controller" of her people, the Eymorg. The Eymorg themselves don't know how their technology works. Kirk, however, has established a communication link with Spock's brain and finds the "Controller". Kara shows Kirk the "Teacher", a device that temporarily gave her the knowledge to remove Spock's brain, but she again denies any help. McCoy uses the "Teacher" and partially restores the brain; but when he begins to forget how the procedure works, Spock himself has to assist him.

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