Revenge of the Cybermen, Part One

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.