This episode has some aspects of "The Man Trap," the first episode of the original series to be broadcast. In that episode a being is capable of altering its appearance in order to accomplish nefarious acts.
While patrolling the Romulan Neutral Zone, the Enterprise encounters a man that has been lost for years and beam him aboard the Enterprise. He appears to be Carter Winston, a well-known philanthropist that once used his fortune to buy food for a colony that was on the brink of starvation. His former fiancé, security officer Lieutenant Anne Nored, is onboard the Enterprise, but Winston jilts her, telling her that it is over between them.
Winston is in fact a Vendorian, a creature capable of assuming any shape of equal size. He is a Romulan spy, sent to the Enterprise to impersonate Captain Kirk and order the ship into the neutral zone. Once there, it can be intercepted by the Romulans and taken as a prize. However, the Vendorian's long association with the real Carter Winston has impressed many of his traits on it, including his love for Nored.
When the Romulans attempt to capture the Enterprise, the Vendorian assumes the form of a missing deflector shield, saving the Enterprise from being captured. The Vendorian then willingly is arrested as a spy and accepts its fate. At the end, Nored agrees to be his jailor and there is the potential (re?)kindling of an interspecies romance.
As humans move out of the solar system, they will no doubt encounter other beings, most of which will be quite strange and bizarre. Of course, humans will be strange and bizarre to them as well. In this episode, the prime alien is capable of absorbing human traits along with assuming the form. Of all the speculated forms that aliens could take, an intelligent shape-shifter is one of the most plausible. On Earth we have chameleons that can change color and other creatures that can dramatically alter their appearance.