An alien named Lazarus is somehow responsible for a kind of "winking out" effect - all of reality seems on the verge of collapse. He is fleeing a monster whose intent is nothing less than the destruction of reality. But through a mishap, Kirk is transported to the home of that monster. And he looks exactly like Lazarus! Kirk must decide which of them - if either - is the monster. If he guesses wrong, it's the end of everything.

Approaching the planet Deneva, the Enterprise encounters a private spacecraft whose pilot is flying desperately into the sun, the apparent act of a lunatic. Spock reveals disturbing news: a pattern of mass insanity that has spread across the galaxy in a nearly straight line, the current end of which is Deneva. The landing party discovers the horrifying truth - and one of them becomes its victim. Now the crew must devise a cure before Spock dies, or is driven hopelessly insane.