Parts: 4
Writer: Kit Pedler
Airdates: 11 February 1967 - 4 March 1967
Production Code: HH

Doctor: second
Companions: Ben, Polly, Jamie

Creature Feature: Cybermen

Story: En route to Mars, the TARDIS is pulled off course by two hundred million miles and winds up on the moon. When the crew ventures out onto the moon's surface to investigate, they find a dome containing a weather control centre, responsible for monitoring and maintaining climactic conditions on Earth. Its source of power is the Gravitron, a machine which uses the power of gravity to affect the Earth's tides and therefore its weather, and also the cause of the TARDIS' detour.

But the weather control centre has begun to encounter problems - a mysterious plague is causing the personnel, including the dome's doctor, to drop like flies. And someone is monitoring all communications between the moonbase and the Earth. As more of the crew become incapacitated, and the base begins to suffer power fluctuations, their control over Earth's weather patterns becomes more and more tenuous. Unless the problems are corrected, the storms unleashed on Earth could devastate half the planet.

Most disturbingly of all, some of the plague victims have been disappearing from the sickbay...and Polly thinks the culprits may be Cybermen. In order to save the Earth from destruction, the Doctor and his companions will have to solve several mysteries. What is the cause of the mysterious plague? What has happened to the missing crew members? And, above all, why is the moonbase important to the Cybermen's plans?