Let it be known- The Enemy Within is not great art. But it does come close to being a great Star Trek episode. Mostly because, in a way, splits down the path of William Shatner as an actor, or how much of an actor he can ever be. After a faulty teleportation back up to ship, ol Kirk is now two, with a doppelganger that beats people to a pulp and makes crude sexual advances to a worker on the ship (probably one of the first instances of attempted rape shown on prime-time TV at the time), while the other 'real' Kirk has lost his will to lead- he's unable to make decisions because his 'evil' side is really the one that takes charge to lead. But what to do as Soolu is stuck on an icy planet? This is all the stuff of pot-boilers, where you keep watching just to see what will make the Kirk's tick- or not- next. It's utterly classic comedy (maybe unintentionally so, maybe meaning to be) as 'evil' Kirk tries to take over the ship incognito, but gets caught when he tries to get the ship away from Soolu and the others!
But then there's the existential crisis of 'good' Kirk, as he doesn't have any real resolve as a leader, except in regards to putting himself, literally it seems, back together. In a way it's kind of trashy, and I could see why one or two Trekkies might find it sub-par. But it does give Shatner lots of freedom as an actor, unlike most of the time where he inspires the best in imitators, and I liked the dichotomy of the psychology present, even as dated as it is. How does one balance the good with the bad? Control, it seems, at least is you're James Tiberius Kirk- which includes libido.