It's "trouble with the transporter" time, as a malfunction with the device causes Kirk to "split into two", with an alternate version of the Captain, all flaws brought to the forefront, running amok on the Enterprise. Not only does the crew have to figure out what to do about the "imposter", but they HAVE to get that infernal transporter working again SOON. Sulu and three other crew members are stranded on a nearby planet, coming perilously close to freezing to death.
The ingenious script is by noted genre author Richard Matheson, which memorably touches upon the provocative and not unreasonable idea that we ALL have different aspects to our selves, the positive and the negative alike, and need them to function as whole beings. Spock is the one to address the concept, since he himself is the product of a human-Vulcan union, and has two sides in opposition to each other. Kirk realizes that without his alternate self, he runs the risk of being very weak and inefficient, and can't continue to function properly as a leader.
This is the core of a story that is both intelligent and wickedly entertaining, with Shatner having great fun at portraying the more emotional, unrestrained half of Kirk. This Kirk even attempts to amorously force himself on Yeoman Rand. Left unchecked, he could have continued to cause great problems for the crew. And the tension arising from the desperate situation facing Sulu and the others is deeply felt. The most amusing touch is the fact that a cute little dog is "dressed up" to look like a diminutive creature that itself is "split", and is the first evidence that Kirk and others have as to what is going on.
Directed by actor / director Leo Penn, the father of Sean and Christopher Penn.
Eight out of 10.