1.05 The Enemy Within

A transporter accident splits Kirk into two separate beings - one good, one evil. Whilst the evil half of the Captain runs riot on the ship committing violent acts, the good Kirk is weakening and finds himself unable to make command decisions. Scotty has a solution in mind, but there's no guarantee the Captain will survive.

The Enemy Within
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TOS Season 1, Episode 5

Richard Matheson
Leo Penn

First Aired Oct 6, 1966
Stardate 1672.1