Strange things happen to a landing party consisting of Jackson, Sulu, and Scotty when they beam down to planet Pyrus 7. Sulu and Scott disappear, then Jackson returns to the Enterprise and promptly dies without a readily apparent cause. A strange voice coming from Jackson's mouth then tells Kirk that there is a curse on the Enterprise, which must leave or the crew will all die.
When Kirk, Spock, and McCoy beam down to investigate, leaving DeSalle in command and Chekov as his sidekick, they are enveloped in fog. They also detect multiple life readings, despite the fact that the Enterprise's sensors detect only the landing party. The landing party subsequently encounters three witches and a medieval castle. Upon entering the castle, they are confronted by a black cat. While exploring the cobweb-draped interior, they fall through a hole in the floor which opens up beneath them and then suddenly find themselves held in irons in the castle's dungeon. Sulu and Scott, now both Zombies, release the prisoners and begin leading them away. When Kirk attempts to overpower them, they all materialize in front of the robed Korob and his ``precious'' black cat whom he appears to consult for advice.
Spock questions Korob as to why previous surveys have not detected life on the planet, and Korob admits that he is not native to the planet. Korob makes a meal magically appear, but Kirk, Spock, and McCoy profess not to be hungry. He then makes plates full of gemstones appear and offers them as an incentive for leaving without further inquiry, but Kirk claims that the Enterprise can manufacture them in bulk and is not impressed. The cat leaves, and shortly thereafter Korob's colleague Sylvia appears. However, around her neck is the same diamond pendant that had been worn by the cat. Kirk overpowers the Zombie Scott, takes his phaser, and demands that Sulu and Scott be released and that their weapons and communicators be returned. Sylvia responds by waving a model of the Enterprise over a candle. The real Enterprise experiences extreme heat, and narrowly avoids burning up when Kirk removes the model and offers to cooperate. To prevent interference from the Enterprise, the model is encased a glass, surrounding the actual Enterprise with a force field it cannot break free from.
Kirk and Spock are chained again in the dungeon, and McCoy is added to the collection of Zombies. Sylvia is meanwhile enjoying the sensual pleasures of her new existence, and mocks Korob for being afraid to use his power when he complains that she is abusing the specimens. Kirk pretends to fall in love with Sylvia in order to pump her for information. She responds by changing into a number of different attractive forms. Kirk learns that Korob and Sylvia have been sent to study the galaxy, using a device called the transmuter. However, Sylvia discovers by probing his mind that Kirk is manipulating her, and becomes angry.
Meanwhile, the Enterprise is breaking free through the efforts of DeSalle. Korob releases the Enterprise from its encasement, then sets Kirk and Spock free and tells them to leave immediately because be can no longer control Sylvia. Sylvia changes to a giant black cat, attacks Korob, and attempts to kill Kirk and Spock. Kirk grabs Korob's scepter, which he suspects to be the mysterious transmuter. After fending off attacks from McCoy, Scott, and Sulu, Kirk tells Sylvia that he has the scepter. Sylvia tries to finagle it away from him, but he breaks the scepter in front of Sylvia. The castle vanishes, and Sylvia and Korob appear as the bizarre blue and yellow puppet-like alien beings they are. Unable to survive in this galaxy without the transmuter, they shrivel up and melt away and Sulu and Scott are returned to normal.