The Time Lords ground the good Doctor Who (Jon Pertwee) on Earth, halting his travels through space. But he quickly finds a new adventure in a crop of mysterious meteorites that seem to signal the start of something big. The UNIT's Brigadier Alistair Gordon Lethbridge-Stewart (Nicholas Courtney) enlists the doctor -- with the help of leggy scientist Liz Shaw (Caroline John) -- to stop a menacing army of plastic robots before it's too late.