Serial: 4D  Episode Nos. 398, 399, 400, and 401.  Title: Revenge of the Cybermen

The Doctor and his friends return to Nerva Beacon to find most of the crew dead of what the crew believes is a plague.  It doesn't take the Doctor long to discover that it is in fact poison being spread by Cybermats sent by the Cybermen, for nearby Nerva is all that remains of Voga, the Planet of Gold that supplied the arsenal that led to the defeat of the Cybermen in a vast war.  It's destruction is the first step in their campaign to again conquer space...

starring Tom Baker as Doctor Who, Elisabeth Sladen as Sarah Jane Smith, and Ian Marter as Harry Sullivan. Written by Gerry Davis, Produced by Phillip Hinchcliffe, and Directed by Michael E. Briant.

Originally transmitted from 19 April 1975 to 10 May 1975 on BBC1.