While patrolling near the Romulan Neutral Zone, the Enterprise discovers a small, badly damaged ship and beams aboard its lone survivor, who appears to be Carter Winston, a wealthy space trader known throughout the Federation for his many selfless acts of philanthropy — and who had gone missing five years earlier. Winston explains that he had been severely injured in an accident, but that a kindly alien nursed him back to health over the past several years. By coincidence, Winston's fiancée, Lieutenant Anne Nored, is serving aboard the Enterprise; when they reunite, however, Winston breaks her heart by informing her that he has changed and can no longer follow through on his plans to marry her. Winston then enters Kirk's quarters and transforms into a bizarre, tentacled alien that renders Kirk unconscious before changing shape again — this time taking on Kirk's likeness.
Posing as Kirk, the creature goes to the bridge, where he orders Sulu to take the Enterprise to the planet Rator III, inside the Neutral Zone, even though doing so would be a serious treaty violation with the Romulan Star Empire. Later the real Kirk awakens and orders the Enterprise diverted away from the Neutral Zone. Now aware that an imposter is aboard, Kirk and Spock discover that Dr. McCoy has also been impersonated, and Anne Nored reports of "Winston's? strange behavior. Kirk and Spock discover that the man they'd thought was Carter Winston is actually a member of the shape-shifting Vendorian race, a species known for being deceitful; as several Romulan ships appear and surround the Enterprise, it becomes clear that the Romulans sent the creature to lure the starship into the Neutral Zone in order to provoke a conflict with the Federation.
Still at large aboard the ship, the Vendorian knocks Chief Engineer Scott unconscious and sabotages the Enterprise's shields, yet confesses to Anne Nored that he actually loves her, apparently having absorbed a great deal of the emotions of the real Carter Winston during the Vendorians' failed attempts to save the injured human's life. The Romulans attack the unshielded Enterprise, whose shields miraculously return to operation in the nick of time; because of his love for Anne Nored, the Vendorian takes on the form of the appropriate deflector shield components in order to protect her, allowing the Enterprise to fight long enough to drive the attacking Romulans away. Although the Vendorian knows that he must stand trial in the Federation for his earlier acts of sabotage and impersonation, he begins seriously thinking about Anne Nored's suggestion that he consider making a life for himself on Earth, as a human.