On patrol near the Neutral Zone, the captain is showing increasing signs of stress and tension, snapping at the crew. McCoy is concerned, and has noted that Kirk is resisting any attempts to render a psychological profile. Kirk orders a course directly into the Neutral Zone and on into Romulan space and it becomes clear that he is acting on their own. A cloaked Klingon ship appears before them, and Spock confirms the Romulans are using Klingon design. Two other ships appear, surrounding the Enterprise.
Spock starts to advance a theory of how the ships approached when they receive a signal from Subcommander Tal aboard the lead ship. He calls upon them to surrender but Kirk figures they want the ship intact. Tal notices that Spock is on board and receives a message saying to give them one hour to consider the matter. In the briefing room, Spock explains the Romulans have a new cloaking device that renders their sensors useless. Kirks considers their options and Spock notes that Kirk ordered them in with no orders. Kirk dismisses their concerns and Tal contacts them and offers an exchange of officers so Kirk and Spock can meet with his Commander.
Kirk and Spock go ahead with the transfer after leaving orders with Scotty to destroy the ship if necessary. They meet with the Romulan Commander, a female. She has Spock ushered outside while she talks with Kirk. He claims that instrument failure caused them to cross the Neutral Zone by accident. She doesn't believe him and has Spock brought back in. She asks him to speak truthfully as Vulcans are rumored to do, but he refuses to speak. From that the Romulan Commander there is some secret he's keeping and they violated Romulan space to get the cloaking device. When she threatens to torture Kirk, Spock steps forward to say that the captain has broken under the stress and ordered the ship in under his own initiative, and he is no longer sane.
The Romulan Commander contacts the Enterprise and confirms that Spock has testified that the intrusion was not an accident, but Kirk isn't responsible. She orders Scott to follow them to a Romulan base where they will be processed and released, and Kirk will be held in confinement. Scotty refuses and the Romulan Commander has Kirk taken away while keeping Spock in her quarters. She discusses Spock's relationship with Kirk and his ambitions, and she offers him a ship of his own in return for presenting her with the Enterprise.
In the Romulan brig, Kirk throws himself at the force field. Spock calls McCoy over to tend to him. Meanwhile, the Romulan Commander invites Spock to dinner… alone. As they pass a red door, Spock begins to walk toward it but the Romulan Commander stops him, warning it's a forbidden area. They continue to the brig where McCoy reports that Kirk is suffering from stress and prosecution. The Romulan Commander uses his condition to confirm Spock's statement and insists that Spock take command. Spock agrees but Kirk goes berserk and attacks him. Spock instinctively grabs his face and administers the Vulcan Death Grip, killing him.
McCoy takes Kirk's body over to the Enterprise and Sickbay, where he revives. McCoy explains to Chapel that Spock temporarily rendered Kirk in a deathlike state, and he's now aware that Spock and Kirk are both operating under Federation orders. Kirk orders her to secrecy and explains the masquerade is to provide the Federation with deniability. He then orders McCoy to prepare for surgery. Later, McCoy calls Scotty down to Sickbay to reveal that they've used plastic surgery to transform Kirk into a Vulcan. They have Scotty get a uniform from the Romulan hostages to aid in the new masquerade.
The Romulan Commander offers Spock a Vulcan meal… and other inducements. The others are on the Enterprise waiting for Spock's signal with the coordinates, but Kirk finally orders them to beam him over as close to the cloaking device chamber as possible. Spock hints that he agrees to join with the Romulans and she orders him to take a Romulan security team to the Enterprise and take command. Spock asks for an hour to be with her and she steps out to slip into something more comfortable. As soon as she's gone, Spock contacts Kirk and directs him to the cloaking room chamber he saw earlier. He's forced to cut off when the Romulan Commander returns and they exchange Vulcan/Romulan romantic hand contact.
Kirk makes his way to the cloaking chamber, knocking out a guard. Tal interrupts the Romulan Commander to inform her that they've found a transmission… from Spock. He immediately turns over his communicator and she realizes that Spock has been playing her, and that they're after the cloaking device. Meanwhile, Kirk gets into the chamber and knocks out another guard, then has Scotty beam him over with the cloaking device. They have 15 minutes to hook it into the ship and they hope Spock can buy them enough time.
While Kirk gets to the bridge and has the crew lock on to Spock, the first officer demands the right to make a statement from the Romulan Commander. Believing Kirk is still on-board she grants him the time. Chekov finally manages to lock onto Spock among the Romulan readings, and the Romulan Commander grabs him as they beam him over. The Enterprise leaves at high speed and Subcommander Tal pursues. Kirk shows Tal the Romulan Commander, who orders Tal to destroy the Enterprise. Scotty finishes hooking up the cloaking device and they disappear from the Romulan screens.
Kirk prepares to drop the Romulan Commander off and has Spock escort her to the brig. They briefly discuss their relationship, and what might have been. Spock then returns to the bridge to advice the captain that Vulcan ears on a human aren't aesthetically pleasing, and Kirk heads for Sickbay to get them removed.