Kirk, Spock, and Bones beam down to a planet to investigate the death of a crewman scout who beamed up from the planet, well, dead. What they find is literally a Halloween setting, complete with a ghostly greeting that seems to be straight out of a haunted house parody. Kirk & Co. subsequently locate the missing men from the first landing party—Scotty and Sulu in a catatonic state, under the command of two aliens with a great power of illusion, who have created the whole Halloween setting as a way of learning about humans.
The aliens, Korob (Theo Marcuse) and Sylvia (Antoinette Bower), slowly become at odds with each other, as Sylvia finds herself attracted to the sensations and experiences of humans—manifested, of course, through her physical attraction to Kirk. Meanwhile, the most wooden lieutenant in Starfleet (Michael Barrier) has command of the Enterprise, preventing the ship scenes from being particularly interesting.
"Catspaw" is a primarily gag-driven episode, with a hit-and-miss track record. Kirk's encounters with Sylvia are moderately interesting. But the episode is fundamentally formulaic, and lives and dies on each piece of its fragmented puzzle. As a lightweight adventure, it's okay; just don't look for much substance.