Kirk is preparing to perform a wedding of crewmen Tomlinson and Martine, when Spock reports that they've lost contact with two Earth outposts along the Neutral Zone separating them from the Romulans, a hostile alien race. They're interrupted when an alert is signal after Earth Outpost 4 reports it's under attack.
As they proceed to Outpost 4, the base stops sending a distress signal. Lt. Stiles, the navigator, insists that the Romulans are responsible. Spock explains that the Neutral Zone was established after a long war between the Federation and the Romulans, and treated via subspace radio with no visual contact. A violation of the Neutral Zone would constitute an act of war. Kirk is under specific orders not to violate the Zone no matter what. Stiles warns that the Romulan ships are painted as birds of prey, and admits he lost several family members in the war.
They near Outposts 2 and 3 and find nothing but dust: the asteroids they were built on have been pulverized. As they go to battle stations, Commander Hanson on Outpost 4 contacts them and informs them that the outpost is all but destroyed. The intruder hasn't responded to hails, but simply opened fire with a high-plasma beam. The intruding ship comes in for another attack, appearing out of nowhere, and Hanson switches to visual. The Enterprise is still out of range, and the crew can only watch as the intruder obliterates Outpost 4. They conclude the intruder is using some form of cloaking device, but Spock picks up something on the motion sensors. The intruder proceeds at a leisurely rate toward the Neutral Zone, but they suspect that it isn't aware the Enterprise is approaching. Kirk orders a parallel course. Stiles disagrees and believes there might be a Romulan spy aboard. Sulu agrees and Kirk orders an internal security alert.
Uhura picks up a communication from the intruder and Spock manages to obtain a transmission of the bridge. The enemy commander turns toward the monitor… and has pointed ears and a strong resemblance to a Vulcan. Stiles suggested Spock could translate the signal and Kirk tells Stiles to leave his bigotry in his quarters.
The Romulan Commander wonders if the Enterprise is a reflector sensor but engage the cloaking device again as a precaution. He then notes that one of his officers sent a signal against orders and demotes the man. The Commander's aide, the Centurion, questions his actions and the Commander admits that if he reports back with his intelligence, war will ensue. He questions whether it would be better to be destroyed but admits as a creature of duty he'll carry his orders out.
The Romulan ship proceeds for the Neutral Zone and will arrive in less than an hour. They meet to analyze the material from the outposts, which has almost entirely disintegrated due to implosion caused by a plasma beam. Scotty confirms they have superior engine speed and power, and Stiles argues that they have to attack before the Romulans enter the Neutral Zone. Spock agrees with him, warning that if the Romulans kept their warlike behavior, they post a deadly threat. Spock detects a comet and Kirk prepares to take advantage of it.
The Enterprisecloses in and plots the Romulans' point of entry. Aboard the Romulan ship, the Commander prepares to use the comet's tail to obscure their ship and then turn and fire. However, when they determine the Enterprise is no longer paralleling them, the Commander orders escape maneuvers. The Enterprise swings around the comet's tail and Kirk realizes his opponent has guessed their tactic. He vows not to underestimate them again, and then orders blanket fire. As the Romulan ship takes several glancing blows, the Centurion leaps to the Commander's aide and pushes him out of the way to avoid falling debris.
On the Enterprise, the phaser circuits overload and Spock tries to repair them. The Romulan ship appears and opens fire, and they try to outrun the plasma beam. It begins to dissipate just before it strikes them, and the Enterprise survives the impact. Spock restores the phasers and relocates the Romulan ship, back on its own course. The Romulan Commander tends to the dying Centurion and is informed the Enterprise is still following
As the Romulan ship approaches the Neutral Zone, Kirk orders the Enterprise to attack and informs Starfleet he's going into the Neutral Zone on his own authority. Aboard the Romulan ship, the Commander realizes that the Centurion is dead and tries to anticipate Kirk. He comes up with an idea and orders all debris into the disposal tubes, along with the Centurion's body.
Aboard the Enterprise, Spock's detects the debris but concludes it's of insufficient mass to indicate serious damage. Kirk figures the Romulan ship has shut down to avoid detection, and cuts power to the ship. The two ships engage in a waiting game and Kirk worries about the lives he's put at risk. McCoy assures him not to destroy himself worrying.
On the bridge, Spock accidentally activates a circuit. The Romulans pick it up and move in, and Kirk anticipates the Commander and opens fire. The Romulan Commander releases more debris with a nuclear warhead concealed inside. Kirk orders the Enterprise away and opens fire, but the detonation staggers the ship. The Romulan Commander refuses to move in for the kill and orders a course for home.
The Enterprise is still intact but only the forward phasers are working and Tomlinson is the only one able to work the controls. Stiles goes to help him, while Kirk orders the ship to play dead. Aboard the Romulan ship, the Commander gives in and prepares to attack, but on his orders.
Spock reports to the phaser control room to assist in making repairs. Stiles tells him to leave while he and Tomlinson make repairs. However, Stiles notices a phaser coolant leak and Tomlinson tries to repair it. The Romulan vessel becomes visible and Kirk orders phasers to fire. Nothing happens and Spock hears the orders. He runs back to discover the phaser control room filled with coolant fumes. Spock activates the controls.
The Romulan Commander opens communications but refuses to surrender. He insists it's his last duty and expresses his regrets, then activates the self-destruct.
In Sickbay, Stiles is recovering and thanks Spock for saving his life. Tomlinson died and his bride is in the chapel. As Kirk leaves, Rand reports that they've received a response and Starfleet will support whatever decision he makes. In the chapel, Kirk hugs Martine and admits it doesn't make sense, but says that there was a reason.