When Captain Kirk and his landing party arrive on Pyrus VII, they are met by eerie mists, a dark castle, wailing witches, zombies and a black cat. They soon learn that they are under the influence of a wizard, Korob, who tries to bend them to his will. They also soon learn that the black cat they saw is more than she appears and is in fact a powerful witch in her right. This beautiful witch, Sylvia, who wears a diamond pendant on her black dress, explains that they are explorers from another galaxy. However, Kirk and Spock must find a means to escape their grasp before they return to the Enterprise.