Crewman Jackson
Jimmy Jones

Lt. Vincent DeSalle
Michael Barrier

Sylvia
Antoinette Bower

Korob
Theo Marcuse

Lt. Kyle
John Winston

The Enterprise loses contact with a landing party led by Sulu and Scotty that has beamed down to planet Pyris VII. One member, crewman Jackson, manages to get through and is beamed back up but dies on the transporter pad. A strange voice is heard warning them to go away but, despite this, Kirk takes Spock and McCoy down to the surface to search for the others. The surroundings are distinctly spooky and three witches appear with another chilling warning for them before vanishing.

The landing party eventually find a mediaeval-style castle and start to explore, but the floor collapses beneath them. When they awake they find themselves in a dungeon, chained to the wall. Sulu and Scotty make an appearance but are apparently in some kind of a trance. They unchain Kirk, Spock and McCoy and take them to meet a being of humanoid appearance called Korob who is dressed like an old-fashioned wizard and is accompanied by a cat which he converses with.

Korob materialises a banquet and a vast wealth in jewels to persuade the Enterprise crew to leave but they refuse his bribe. At this point the cat leaves the room, reappearing in the form of a gorgeous woman called Sylvia. Kirk threatens them with a phaser that he grabs from Scotty but is forced to back down when Sylvia holds a miniature Enterprise over a candle flame. On board the starship the temperature starts to rise dramatically as the landing party are taken back to the dungeon.

Shortly afterwards, Kirk is summoned by Sylvia. She tells him that she wants him to help her to explore the joys of her physical existence. He starts kissing her, but when she realises that he is only using her she flies into a rage and dismisses him. A frightened Korob releases the landing party from the dungeon, telling them that Sylvia has gone insane. They all try to escape from the castle but are stalked by Sylvia in the form of a monstrously huge black cat.

Korob is hit by one of its paws and loses his 'magic wand', which Kirk grabs and, despite pleas from Sylvia, smashes to pieces on the floor. The 'wand', as Spock had suspected, was apparently a kind of transmitter which Korob and Sylvia used to amplify their thoughts and change the nature of matter. The castle disappears instantly and Scotty and Sulu are released from their trance. On a nearby rock, two small blue creatures whine pitifully as they die, unable to survive on the planet in their natural form.