A transporter malfunction splits Kirk into a gentle half and a savage half, neither able to command alone.
A transporter malfunction creates two versions of Captain Kirk, one gentle and indecisive and the other violent and impulsive. While the savage Kirk terrorises the ship, the crew discovers that both halves are needed, and the transporter itself is not working properly.
This story is a fan favourite for its exploration of how command requires both compassion and aggression. William Shatner plays both halves, and it gives Spock one of his early significant moments of insight. It also establishes the transporter as a source of danger as well as convenience.