is patrolling the edge of the Neutral Zone and detects a one-man vessel damaged by a meteor swarm. Spock identifies the vessel as belonging to Carter Winston, a philanthropist space trader who has been missing for five years. They beam the survivor aboard and he is indeed the famous Carter Winston. McCoy explains that his daughter was on Cerberus ten years ago when a crop failure occurred and Carter personally financed enough food to get the planet through its crisis. Spock informs Carter that his fiancée Lt. Anne Nored, a security officer, is aboard. He requests Carter's identity tapes and notes that an identity check and medical examination are required. Carter readily obliges and in the briefing room, Kirk and Spock confirm that Carter is who he claims.
In Sickbay, McCoy notices some odd readings from Carter but assumes there's instrument malfunction. Nurse Chapel arrives and informs them that Anne is waiting for them. She comes in and the medical officers leave them alone. Carter explains that he intended to marry her, but on his last trip he crashed on Vendor. He says that he's changed after a number of complicated operations and was assigned one of their people to watch over him. She doesn't understand how he's changed but Carter says he can't explain and has to break off their engagement.
Kirk goes to his quarters to prepare a report on Carter's rescue. Carter visits him but when Kirk turns back to his screen for a minute, "Carter" transforms into a non-humanoid alien life form and renders Kirk unconscious with a touch. The alien then takes on the appearance of Captain Kirk and goes to the bridge. In Sickbay, Chapel reports that there's no indication of instrument failure and McCoy determines to find the answer.
On the bridge, "Kirk" orders a course through the Neutral Zone and both Spock and Sulu are surprised. Sulu warns that the Romulans can confiscate their ship under the treaty. Spock reports that there is something, possibly a ship, at extreme long range. He warns against trespassing but "Kirk" says that Carter has told him there is a planet it's vital for them to get to as quickly as possible to save the entire planetary population. They lay in the course and "Kirk" leaves the bridge. A little later, the real Kirk wakes up and goes to the bridge, and is surprised to discover that he apparently dozed off. Kirk is surprised to discover they're in the Neutral Zone but Spock plays back the bridge logs showing that the captain gave the order. They set course out of the Neutral Zone and go to yellow alert. Kirk and Spock go to Sickbay to have the captain tested.
In Sickbay, McCoy informs Carter about the test results but the alien transforms to its natural form and renders him unconscious. A minute late, Lt. Nored comes in and asks for advice about Carter, and "McCoy" advises her to accept what Carter has said and agree to break off the engagement. They're interrupted when Kirk and Spock arrive. Lt. Nored leaves and Kirk asks for a complete medical examination. "McCoy" tells him he'll have to do it later and Spock, suspecting is something up, asks if he have made an error. "McCoy" readily admits it's possible and Kirk says they'll come back later. In the hallway, they both realize that McCoy would never agree with Spock about his being in error. They go back in and McCoy wakes up in the laboratory. They realize something is wrong in the laboratory and Kirk confronts one of the three examination tables. Kirk notes there were only two examination tables in the room, then takes a bottle of Orientine acid that will burn through almost anything. When he threatens to pour it on the examination table, the alien transforms to its natural form. Spock realizes it's a Vendorian from the quarantined planet of Vendor, and the natives have the ability to shapeshift. They practice deceit as a way of life, putting their planet off-limits. The Vendorian attacks them and flees into the hallway. Spock sends out a security alert and explains that the intruder can shapeshift.
The Vendorian reverts to its Carter Winston form but Anne confronts him, realizing who the only stranger on board is. "Carter" manages to disarm her for a brief second and she hesitates before shooting her. Kirk arrives and asks why she paused, and Nored admits she couldn't harm the image of the man she loved. Nored admits she realizes that now. The ship goes to red alert as two Romulan battle cruisers approach. Kirk tries to explain the situation to the Romulan Commander, who insists that they abide by the treaty and surrender the ship. Kirk asks for a few minutes to inform the crew and the Romulan Commander agrees. No one has located the Vendorian yet. Kirk figures that Carter's ship was bait and the Vendorian is working as a spy for the Romulans. Kirk contacts the Romulan Commander and informs him he refuses to surrender. The Romulan Commander denies employing a spy and prepares to attack.
In Engineering, Scotty finds a crewman sabotaging the controls. The "crewman" renders Scotty unconscious and continues its sabotage. The deflector shields go down and Kirk contacts Gabler in Engineering. The shuttlecraft hangar doors open and Kirk has Sulu override and close the doors. Kirk goes to Engineering and Scotty explains that it'll take two hours to fix the controls. In the hangar bay, Nored confronts "Carter," and the Vendorian talks about how Carter described his deep feelings about Nored. It explains that Carter crashed on Vendor and that it tended to him. Carter took a year to die and it gained more of Carter's emotions and memory the longer it stayed in Carter's form. The Vendorian came to love Nored as Carter did, feeling Carter's emotions. Nored drops her guard and the Vendorian reveals its true form, and asks if she can love an alien. He reverts back and explains that's why he told her to forget him. Kirk arrives but the Romulans open fire and the Vendorian escapes.
Kirk returns to the bridge and gets a damage report. The Romulan Commander repeats his surrender request and all looks hopeless, but a deflector shield comes up. Sulu fires on the lead Romulan vessel and they take a hit on the deflector shield. They open fire on the second ship and both ships retreat. Kirk thanks Scott, who notes he hasn't repaired the ship yet. They conclude that the Vendorian took the place of the deflector shield. The Vendorian arrives on the bridge and apologizes for endangering the ship. He explains that he is a lower order outcast among his people, and the Romulans offered him something to do with his life. The Vendorian admits that Carter Winston loved life and loved Nored, and the Vendorian shared the same feelings. Kirk says the Vendorian will have to stand trial but its actions to save the Enterprise
will be taken into consideration. The Vendorian reverts to its Carter form and Nored requests the assignment to guard the prisoner. "Carter" is surprised but Nored says that form doesn't matter and wants to talk about the possibility of a life together. Kirk agrees and the couple leave the bridge. McCoy is relieved they caught the spy and Spock notes that having two Dr. McCoys on the ship might have raised medical efficiency to acceptable levels.