Star Trek: Season 2, Episode 17

2 - 17: A Piece of the Action

The Horizon visited the planet Iotia a hundred years ago, long before the Prime Directive forbid interference in pre-warp alien cultures. Beaming down to investigate the damage, Kirk, Spock and McCoy discover a culture run by mobsters! It seems that Horizon left behind a book, "Chicago Mobs of the Twenties", and the highly imitative people of Iotia patterned their society after the groups described in its pages. How can Enterprise undo the grievous harm Horizon did to this civilization?