Kirk and the Enterprise try to make contact with the people of Sigma Iotia II who have modeled their culture around the a book "Chicago Mobs of the Twenties" left behind by another Federation starship. Kirk and a landing party beam down into the middle of early twentieth century gang war and are taken prisoner after following the Prime Directive and not supplying weapons to the people.

The TARDIS materialises in 21st century Earth, where the Doctor discovers that he's the exact double of the philanthropic scientist Salamander. Salamander has ended world hunger and has been supplying humanitarian aid to parts of the world which have recently suffered inexplicable seismic activity. However, his former associate, Giles Kent, claims that Salamander is in fact a power-crazed maniac who's slowly replacing the authorities of Earth with lackeys under his thumb. The Doctor isn't so sure, and agrees to impersonate Salamander and find out the truth only if Jamie and Victoria can prove to his satisfaction that Salamander is indeed an evil man. However, Jamie and Victoria soon find themselves in over their heads - and even before he has a chance to act, the Doctor is already being hunted by Donald Bruce, the suspicious chief of World Zone security.