First contact becomes an interstellar incident when the Starship Enterprise responds to a distress call from an unknown ship and saves the life of a man left to die by his own people. Berlis, member of a telepathic species calling themselves the Isitri, claims not to know why those from his homeworld want him dead. Captain James T. Kirk wants to believe him, but the damage is done: the Enterprise can neither leave the stranger to die nor turn him over to those who would kill him.
Berlis seems harmless, but his people say he cannot live among them: his telepathy is so strong that their wills are subsumed to his. The same fear that compels the Isitri to seek the death of one of their own drives the neighboring Odib people toward genocide. For every time a "troublesome mind" dominates the Isitri, the Odib pay the price in their own blood.
With Spock becoming erratic under Berlis's influence, and the Isitri begging Kirk to allow them to destroy the man who threatens their existence, matters take a disastrous turn when Berlis makes his way back to Isitra...and an entire world falls to his whims.