The marriage of Angela Martine and Robert Tomlinson is interrupted when a Romulan warbird attacks and destroys Outpost 4, which guards the Neutral Zone between Federation and Romulan space. Kirk learns that the Romulan ship has also destroyed three other outposts and is now running at full speed towards home.
The U.S.S. Enterprise pursues the warbird, hampered by the fact that the Romulans have constructed an invisibility screen which shields them from view. While the screen protects the Romulans from detection visually, it also prevents them from using their weapons or visual aids. The Romulan Commander, therefore, isn't sure whether his radar is detecting a Federation ship in pursuit or a harmless space echo.
The U.S.S. Enterprise is able to pick up a visual from the Romulan bridge, which shows the previously unseen Romulans to look very much like Vulcans. This sparks an old prejudice in Lieutenant Andrew Stiles, whose family fought in the Romulan wars. He is instantly suspicious of Spock, whose physical characteristics are remarkably similar to the Romulans they are chasing.
After every attempt to lose the U.S.S. Enterprise fails, the Romulan Commander is forced to turn and fight. Both ships are damaged; when the U.S.S. Enterprise's phaser banks are damaged, they emit a poisonous gas which disables Stiles and Tomlinson, who are manning the weapons. In a tense race against time, Spock manages to fire the only remaining phaser manually, disabling the Romulan ship. The Vulcan manages to save Stiles, but Tomlinson is killed. Stiles, realizing by Spock's actions that his bigotry was misplaced, admits that he was wrong.
The Romulan Commander contacts the U.S.S. Enterprise and, in a stirring moment, tells Kirk that under other circumstances, he suspects they might be friends. Rather than let himself and his ship be taken prisoner, the Romulan self-destructs the warbird.