Despite the earlier Romulan war, the Romulans have never been seen in person. The Enterprise, in pursuit of a Romulan aggressor, makes a discovery that causes suspicion about a senior crew member.

Encountering an uncharted planet in an empty region of space, Kirk and his crew are entertained by Trelane, a playful being who lacks the self-control to wield his great psionic powers with discipline.