The Enterprise is redirected to investigate the loss of communication with the Gamma 7A star system. While in route, Spock senses the destruction of a Vulcan manned starship, the Intrepid. Spoke says that all the Vulcans aboard have been killed. The Enterprise is ordered to Sector 39J to investigate what may have happened with the Intrepid after communication is lost with them as well.

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.