The Enterprise encounters a planet recently visited by another starship and discovers that a book on the Chicago mobs of the 1920s accidentally left behind by a crew member of the previous ship has become the basis of the planet's society structure over 200 years of the planet's time (the starship having had warp drive). The intelligent but imitative inhabitants show a keen interest in replacing telephones with communicators and replacing tommy guns with phasers when Kirk, Spock and McCoy - along with their standard Starfleet landing party equipment - are captured.
