Two aliens in a spooky castle terrorise the landing party in order to understand human fear.
After a landing party on Pyris VII loses contact, crewman Jackson beams back dead, his body delivering a warning that the ship is cursed. Kirk, Spock and McCoy go down, ignore three spectral witches, enter a gothic castle, fall into a dungeon and find Scotty and Sulu under mental control. Their captors are Korob and Sylvia, aliens from another galaxy who control matter with a device called the transmuter. Sylvia holds a model of the Enterprise over a candle to heat the real ship, then traps it inside a crystal.
Sylvia takes different forms to tempt Kirk, who plays along to learn about her. Korob tries to release the humans but is attacked by Sylvia in the form of a giant cat. Kirk seizes the transmuter wand and smashes it, dissolving the castle and the illusions, and the aliens are revealed as tiny creatures who die without it. Robert Bloch wrote this Halloween episode, the first filmed with Chekov.