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The Doctor, having had his regeneration forced by the Time Lords (see The War Games), has been exiled to Earth. A swarm of meteors falls on the English countryside, and a poacher discovers a mysterious plastic polyhedron at the crash site. In the meantime, Brigadier Lethbridge-Stewart of UNIT is trying to recruit Dr. Elizabeth "Liz" Shaw as his scientific advisor and investigate the unusual meteor falls.
The plastic polyhedron is actually a power unit for a non-physical alien intelligence known as the Nestene Consciousness. Normally disembodied, it has an affinity for plastic, and is able to animate humanoid facsimiles made from that material, known as Autons. The Nestene have taken over a toy factory in London, and plan to replace key government and public figures with Auton duplicates. The Auton in charge of the factory sends other, less human-looking Autons to retrieve the power units from UNIT and the poacher.
The Doctor discovers that his TARDIS has been disabled by the Time Lords and he is trapped on Earth. He convinces Lethbridge-Stewart that he is the same man who aided him before to defeat the Yeti and the Cybermen, despite his change in appearance. Together with Liz, he uncovers the Nestene plot, just as Auton mannequins are activated across London and start killing people. However, the Doctor creates an electroshock device that he believes will disable the Autons.
UNIT attacks the plastics factory, but the Autons are impervious to gunfire. The Doctor and Liz make their way inside and encounter the octopus-like plastic creature that the Nestene have created with the power units as the perfect form for the invasion. While the Doctor struggles with the creature, Liz manages to use his machine to shut the creature down, and all the Autons "die" as well, being part of the Nestene gestalt consciousness.
The Brigadier fears the Nestene will return and asks for the Doctor's help. The Doctor agrees to join UNIT in exchange for facilities to help repair the TARDIS, and Liz stays on as his assistant.
| Preceded by: The War Games | List of Doctor Who serials | Followed by: Doctor Who and the Silurians |