TOS: The Enemy Within

A transporter accident leaves the Enterprise with two Kirks; one good, one evil. It takes the crew some time to figure out they are dealing with two Kirks, and the scratches on evil Kirk's face ( courtesy of Janice Rand) is only a good indicator until Evil Kirk figure out he just needs to scratch up Good Kirk. They missed the most important clue- Evil Kirk wears a ton of eyeliner, and Good Kirk wears none. This episode looks at our human dark and good side, and why we need both side by side.