TAS Season 1 Episode 6 The Survivor

Plot

While patrolling near the Romulan Neutral Zone, the Federation starship Enterprise finds a small private ship flown by a Vendorian, an alien species that can transform its shape at will. The alien dupes the Enterprise crew by assuming the form of Carter Winston, a Federation citizen and philanthropist who has been missing for five years. Winston's fiancee, Lieutenant Anne Nored, happens to serve as a security officer aboard the Enterprise; upon their reunion, he breaks off their engagement without explanation.
The Vendorian renders Captain James T. Kirk unconscious, takes his form and orders the helmsman, Lt. Hikaru Sulu to steer the Enterprise into the Neutral Zone, where Romulan warbirds lie in wait. The real Kirk regains consciousness, however, he is bound and gagged. Meanwhile, Dr. McCoy and Spock become suspicious when "Dr. McCoy" allows that he might have made a mistake - something the real McCoy would never admit to - and a new examination table materializes in the medical bay. They force the Vendorian to reveal his true form, and the alert is sounded. The Vendorian escapes detection and disables the Enterprise's deflector shields, leaving it vulnerable to Romulan attack, while its presence in the Neutral Zone gives the Romulans a pretext to destroy the Enterprise. Ursula hears Kirk rummaging around; she releases him. He then explains that the Vendorian was in league with the Romulans from the beginning.
The Vendorian takes the form of a deflector shield around the Enterprise and the Romulans retreat. The Vendorian shows himself to the Enterprise crew, and explains that the nature of his kind is to gradually assume the memories and personal traits of those they impersonate. Because he spent too much time in the form of Carter Winston, he became unwilling to let the Enterprise crew be killed. He is arrested and will face trial, but Kirk tells him that his actions to protect the Enterprise will be taken into consideration. Nored volunteers to guard the Vendorian, saying that he has become similar enough to Winston that she has feelings of love for him.