Seeming tense and erratic, Captain Kirk takes the U.S.S. Enterprise into Romulan space and the ship is immediately surrounded by Romulan warships. Kirk and Spock beam aboard the Romulan flag-ship and confront the Romulan Commander, a woman. Kirk explains that his ship entered Romulan territory because of equipment malfunction. Spock, however, denounces this explanation, saying Kirk ordered them here, due to his reduced mental stability. This, not surprisingly, enrages the Captain. The Romulan Commander orders the U.S.S. Enterprise be taken to the Romulan base. Scotty, placed in command of the starship, refuses. McCoy is beamed aboard the flag-ship to tend Kirk, who has become irrational to the point of violence. When he arrives, Kirk attacks Spock who reacts, without thinking, by using the Vulcan death grip, killing Kirk.
McCoy returns to the U.S.S. Enterprise with Kirk's body, while Spock remains on the Romulan ship. Unknown to the Romulan Commander, this has all been a ploy to sneak the officers on board and steal the Romulan cloaking device. After Kirk's body has been removed to his ship, the Romulan Commander begins to try and entice Spock into defecting to the Romulan side.
Disguised as a Romulan, Kirk returns to the Romulan ship and steals the vessel's cloaking device and returns with it to the U.S.S. Enterprise. When the Commander discovers the theft, she feels betrayed and in retaliation decides to execute Spock. The Vulcan pretends to confess to her and ultimately stalls until Scotty is able to install the cloaking device on board the U.S.S. Enterprise. Spock is beamed back aboard the starship, but since the Romulan Commander was standing near him, she is also beamed aboard.
The Romulan Subcommander, now in charge of the flag-ship, gives chase, ordered by his commander to destroy the Federation ship. Fortunately, the newly-installed cloaking device works and the U.S.S. Enterprise makes good her escape, with the Romulan Commander on board a their prisoner.