Scotty, Kirk, and McCoy beam down to Argellius II for shore leave. Scotty becomes attracted to a belly-dancer and goes on a romantic walk with her. Later the belly-dancer is found stabbed to death... with Scotty holding the murder weapon and claiming to suffer from amnesia. The Argellians are eager to put Scotty on trial but Kirk believes someone else is responsible and holds a hearing to get at the truth.

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.