The Enterprise goes to Deep Space Station K7 responding to a distress call. After reaching the station, Kirk finds out the distress call to be unwarranted and the Klingons have come there looking for some shore leave. Kirk must maintain order as the Federation and Klingon tension is high. That and the fact that the station and the Enterprise are being overrun by small furry animals known as tribbles.

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.