 Kirk and Spock lead a party beaming down to a normally barren planet where a superior life form lures them by images of three witches, a cat and a castle, clearly fashioned to fit human minds. Inside the apparent puppet master Korob soon turns out to be the lesser of the cat, which transforms to her female form, Silvia, who is getting addicted to the earth-concept of luxury, and demonstrates mind-power that can heat up or freeze the whole Enterprise. While McCoy, Scotty and Sulu are totally controlled, Kirk and Spock engage in a desperate battle of wits, as even Korob is frightened to their side by the out-of-control cat-woman, so when she feels scorned...