Star Trek: The Animated Series

The Survivor (1x6)

Air date: Oct 13, 1973

Patrolling near the Neutral Zone, the U.S.S. Enterprise finds a ship and rescues Carter Winston, a Federation citizen, financier, and philanthropist missing for 5 years. However, Winston turns out to really be a Vendorian, an alien species that can transform its shape at will. He is also allied with the Romulans.