Operation -- Annihilate!

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.

Synopsis adapted from TVmaze (CC BY-SA 4.0)