Amok Time

Mr. Spock shows evidence of starting to lose control over his emotions. When he insists that he needs to return home to Vulcan, despite being engaged in an important diplomatic mission, Kirk forces him to admit that Vulcan biology requires him to take a mate. After diverting to Vulcan against Starfleet's direct orders, Kirk finds himself in a life-or-death struggle against Spock, at the instigation of Spock's wife-to-be.