Enterprise visits an idyllic planet, where McCoy encounters a large white rabbit that is "very late". And shortly afterwards, a little girl who is following him. McCoy isn't becoming insane; it soon becomes apparent that thinking about a thing will cause it to materialize, as everyone encounters people and things from their past or their fantasies. But something on the planet's surface is draining power from Enterprise. And then McCoy, convinced that what he sees is a harmless illusion, confronts a charging knight - and dies, pierced through the heart! What kind of odd world is this, and how can the ship and crew escape it?

Within a "star desert" the crew encounter the odd, storm-wracked planet Gothos. Then Kirk and Sulu disappear from the bridge. The planet's sole inhabitant is the foppish Trelane, who lives in a manor house be believes represents modern Earth, but which is actually centuries out of date. He has plans for the crew, and the power to ensure that he gets what he wants. Kirk manages to thwart him, but it's only temporary: Trelane moves Gothos itself to intercept the fleeing starship! Against that kind of power, Kirk will need all his wits...