Carter Winston was the alias assumed by a Vendorian as part of a subterfuge plot. Prior to assuming this false identity, this Vendorian tended to the real Carter Winston after his trading vessel crash landed on the planet Vendor, until Winston's death almost a year later in 2265.
In 2269, this Vendorian began working for the Romulans, in the guise of Carter Winston. As part of the ruse, the impostor set Winston's craft adrift near the Romulan Neutral Zone, where it was eventually discovered by the USS Enterprise.
After successfully convincing the crew he was Winston by producing the Human's identity tape and undergoing an examination by Doctor McCoy, he "reunited" with Winston's fiancée, Anne Nored, telling her that he'd changed and couldn't ever marry her.
Entering Captain James T. Kirk's quarters with the pretense of discussing the condition of his ship, he was able to knock the captain unconscious, lay him on his bed, and assume his form. As Kirk, the Vendorian ordered Sulu to set course for Rator III. When the helmsman and Spock questioned his judgment, as the course would take them through the Romulan Neutral Zone, the impostor defended his own actions, claiming he ("Kirk") had spoken to Winston about it and was satisfied that the action was necessary.
When the real Kirk mistakenly came to believe that the Vendorian's actions had been his (Kirk's) and that his inability to remember them was due to having blacked out, he reversed the impostor's orders and took Spock's advice, heading to sickbay for a complete physical, in case he was still a danger to the ship.
Meanwhile, the Vendorian subdued Dr. McCoy and took on his form, then dragged him away from his work. When Nored came to ask McCoy's advice about Winston, the impostor pretended that he (McCoy) had discussed the situation with Winston and once again pleaded with her to forget the man. After she left, he responded to Kirk's request for a physical by telling the captain that he was too busy and that Winston had tested normal. When Spock pointed out that he ("McCoy") might have made an error, he said there was always the possibility of that and offered to review the results again later. This raised a red flag in both Kirk and Spock's minds.
After the pair of officers found McCoy, Kirk realized there were now three examining tables in the room where there had been two. The Vendorian reacted when Kirk subsequently threatened him with a vial of Orientine acid, the alien finally assuming its own true form. Attacking all three officers, the Vendorian escaped and reverted to Winston's form. When Nored saw him, she recognized him as the intruder that Enterprise's security teams were looking for and aimed her phaser at him, but he knocked it out of her hand before she could fire.
He further assumed the form of a crewman and took apart a panel in main engineering, thereby making Enterprise's shields inoperable. When Montgomery Scott attempted to stop his actions, he subdued the engineer, too. When Scotty regained consciousness, he estimated that the Vendorian's sabotage would take at least two hours to repair.
The Vendorian Winston had retained so many memories belonging to the real Winston that he was unable to complete his mission for the Romulans, having fallen back in love with Winston's former fiancée, who he was now in danger of hurting. To save her and the rest of the crew of the Enterprise, he transformed into a deflector shield and stopped the Romulan attack.
Reappearing in his true form after the Romulans left, he apologized for his actions, saying he was a member of the lowest group in his species and wanted to be valued by someone, a prospect that the Romulans had offered him. Kirk informed him he would have to stand trial, but that the captain would recommend his heroic actions be taken into consideration. He once again took Winston's form, prompting Kirk to place him under guard, much to McCoy's relief. Anne Nored volunteered for guard detail, saying she was interested in talking with him anyway. Kirk granted her request. (TAS: "The Survivor")