The Enterprise investigates the lack of response from Earth outposts 2 and 3 (and subsequently 8) monitoring the Neutral Zone between planets Romulus and Remus and the rest of the galaxy. Using his authority as a ship's captain, Kirk is preparing to solemnize the marriage of crew members Angela Martine and Robert Tomlinson. However, the ceremony is interrupted when Earth outpost 4 reports itself under attack by an unknown space vessel. The Earth outposts have been constructed on asteroids, and were authorized by treaty following the atomic war with the Romulans more than a century earlier. No human or Romulan, however, has ever seen the another. Navigator Stiles is especially bitter because his ancestors were killed in the Earth-Romulan war
As the Enterprise communicates with outpost 4, Commander Hansen reports an attack underway by an unknown weapon from a spaceship which subsequently vanished. Immediately thereafter, the attacking starship becomes visible briefly, destroys the outpost, then vanishes again. However, Kirk is able to track it despite its cloaking device, and flies on a parallel course. Spock is able to lock in to Romulan transmissions, and the crew of the Enterprise gets a glimpse of the Romulans (who have pointy ears!). The anatomical resemblance of Rolumans and Vulcans leads Stiles to question Spock's allegiances.
Meanwhile, the Romulan commander questions his mission of starting a war, and discusses it with his Centurion. Aboard the Enterprise, scanners show debris from outpost 4 contains disintegrated cast roginium, the hardest substance known to science, and Spock surmises that a plasma weapon must have been used. The Enterprise takes advantage of the passage of the Romulan vessel through a magnitude 7 comet tail to attempt an attack. However, the Romulans are planning to turn and attack the Enterprise once the comet rendered its sensors ineffective. Kirk fires phasers at random in hopes of hitting the Romulan and scores a minor hit. Unfortunately, this overloads the phasers and renders them temporarily unusable. The Romulans fire a plasma-ball and the Enterprise retreats at full speed. Fortunately, the plasma-ball dissipates partially before impacting. The Enterprise fires again and again does only minor damage. However, the Romulan ship ejects debris including the body of the Centurion who was killed in the second Enterprise attack.
The Enterprise then sits motionless for 9 hours, 47 minutes hoping the Romulan ship will make a move and reveal itself. Spock accidentally sets off an alarm while fixing the phasers, and the Enterprise begin maneuvering and fires off another salvo. The Romulan vessel then ejects debris, inside of which it has placed a nuclear weapon. Spock detects a metal object, and the Enterprise destroys it before running into it, but 22 casualties, mostly radiation burns, still result. Stiles and Tomlinson are overcome by pink gas in the phaser room, but Spock is able to get there and fire the phasers manually. This is enough to render the Romulan inoperative, and its captain self-destructs. Tomlinson dies, but Spock is able to save Stiles. Kirk must then comfort Martine over the death of her fiance.