Unbeknownst to the Enterprise crew, a transporter malfunction creates a duplicate of Captain Kirk, which somehow receives all of the real Kirk's darker, "negative" qualities. The story documents Kirk and Spock's attempts to track the faux Kirk through the ship as the impostor runs around causing trouble—particularly in one episode where he has a rather nasty encounter with Yeoman Rand.
"The Enemy Within" epitomizes why TOS could be so much fun. We have mood and attitude injected into the anti-Kirk scenes, thanks to a wonderfully bombastic score by Sol Kaplan. We have William Shatner chewing scenery like there's no tomorrow ("I said give me the brandy!" "I'M CAPTAIN KIRK!" "I want to live!") in delightfully entertaining scenes.
And we have an effective balance of good dialog utilizing Kirk, Spock, and McCoy. Keeping the story grounded in the intelligent is the idea of the real Kirk's slow demise of will and his eventual inability to function as captain because he has been drained of the aggression that his counterpart possesses. It's an effective revisit to Jekyll and Hyde lore, and even though it can be campy at times, it's quite engaging along the way.