The Enterprise answers a distress call from Federation Outpost #4, a monitoring station on the Federation side of the neutral zone with the Romulan Empire. The outposts were established over a century ago and no one has actually seen a Romulan. The Romulan vessel seems to have some type of high energy explosive device as well as a cloaking device to make the ship invisible. When it appears that Romulans bear a strong resemblance to Vulcans, Kirk must deal with a rebellious crew member. He must also engage in a dangerous cat and mouse game with a very intelligent Romulan commander.