Apparently acting irrationally, Kirk orders the Enterprise into the Romulan Neutral Zone. They are, predictably, detected, and commanded to surrender or be destroyed.

Kirk and Spock surrender themselves to the Romulans to save the ship, and meet the female Commander of the Romulan vessel. They are accused of attempting to steal the Romulan cloaking device, but Spock asserts that Kirk was not acting under orders–"He is not sane", Spock declares. Spock is believed, because it is rumored–and Spock confirms–that Vulcans are incapable of lying. When Kirk attacks him, Spock defends himself, using, as he says, a "Vulcan death grip".

Now that Spock seems to be coming around to her side, the Commander intends to arrange a romantic interlude between them, as an inducement for him to defect. However, it turns out that Vulcans are not so incapable of lying, after all. Or Spock isn't, anyway. He and Kirk were, indeed, conspiring to steal the cloaking device. It all comes out, in the end, but being the heroes, Kirk and Spock escape, and the balance of power is maintained.