The Time Ring returns The Doctor, Sarah and Harry to Nerva Beacon thousands of years before they left. Where the Doctor, Sarah and Harry find the Beacon's crew have been wiped out by a space plague, only to discover the crew have been murdered by a cybermat and the culprits are the Cybermen and geologist Professor Kellman who bid to wipe out "Voga" a planet of gold, home of the subterranean Vogans who defeated the Cybermen in a war.

The Cybermen force the Doctor, Stevenson and Lester to carry bombs to the centre of Voga while Sarah, Harry and Tyrum decide to contact Vorus.