The crew visits a planet whose culture was modelled on an Earth book about 1920s gangsters.
The Enterprise visits Sigma Iota II to find out what happened to a Federation ship that visited a century before. The planet's people have organised their society after a book about Chicago gangsters. Kirk and Spock are taken into the mob and are told they must work for the bosses.
The episode is a comedy that was written to be funny. Kirk adopts the local speech and drives a car, while Spock reluctantly plays the part of a tough hood. It is fondly remembered as one of the most entertaining of the series, and for its use of cultural contamination.