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.