The Enterprise encounters a strange cube, moving through space without apparent means of propulsion. When it begins emitting deadly radiation, Kirk orders it destroyed. Later, they encounter the ship from which the cube originated: the Fesarius, the flagship of the First Federation. Its commander, Balok, indicates that the Enterprise will be destroyed for behaving violently.

In the end, desperate, Kirk tries bluffing: he claims the all Federation starships contain a substance, corbomite, which will destroy any attacking ship. Balok decides instead to tow the Enterprise, but it is able to escape. In doing so, Balok's ship is--apparently--badly damaged, and when Kirk, Spock, and McCoy transport over to render aid, Balok reveals that it was all a ruse, to test them, and they can look forward to cordial relations.