The Doctor, Harry and Sarah return to the Nerva Beacon, but some time in the past where its use is as a beacon. The beacon is in orbit around an unknown asteroid, known by the natives as Voga - the planet of gold.

The Cybermen plan to destroy the planet, as gold is lethal to them, and use the beacon as a base. To do this they use the Cybermats to spread a disease. Sarah is infected, and the Doctor uses the transmat, which screens for foreign particles, to cure her.

On Voga the Doctor discovers that Kellmen, who was presumed to have been helping the Cybermen, was in fact helping the Vogans, has constructed a giant missile. Kellmen had lured the Cybermen to the base whereby the Vogans' missile would be used to destroy it.

The Cybermen learn of the plan and set the beacon on collision course with Voga, and evacuate to their ship. The Doctor manages to aim the Vogans' bomb at the Cybership, destroying it. The Doctor, Harry and Sarah return to the beacon. The Doctor manages to save the beacon, and when they crew return the Doctor and his companions has already disappeared.