"The USS Enterprise is on a mission to Alpha 177, a planet with a volatile atmosphere. While on the world, Captain Kirk is injured in a fall and is beamed back to the Enterprise. However, due to a transporter malfunction, Kirk is split into two people: one good but indecisive and ineffectual; the other evil, impulsive and irrational.

The good Kirk is taken to Sickbay, where Dr. McCoy attempts to treat him. However, the evil Kirk escapes and begins to cause trouble for the Enterprise. He attacks Yeoman Rand, tries to take control of the ship, and even threatens to destroy it.

Spock and the other crew must find a way to stop the evil Kirk before he can do more damage. They eventually succeed in trapping him in a force field, but they know he will not stay contained for long.

The episode ends with Kirk facing his evil self in a mental duel. Ultimately, Kirk can defeat his lousy self and merge into one person. The episode explores the duality of human nature and the importance of balance.

Here are some additional details about the episode:

The episode was written by Richard Matheson, also known for writing the stories "I Am Legend" and "Duel."
Leo Penn, the father of actor Sean Penn, directed the episode.
The episode features William Shatner in a dual role, playing both the good and evil Kirk.
The attack was nominated for an Emmy Award for Outstanding Writing for a Drama Series.
Critics praised the episode for its suspenseful plot and Shatner's performance.
"The Enemy Within" is a classic episode of Star Trek that explores the duality of human nature. It is a suspenseful and thought-provoking episode that will entertain and challenge viewers."