Parts: 4
Writer: Gerry Davis
Airdates: 19 April 1975 - 10 May 1975
Production Code: 4D

Doctor: fourth
Companions: Sarah, Harry

Creature Feature: Cybermen

Story: The Time Ring brings the Doctor, Sarah and Harry back to the Ark - but things have changed. The TARDIS is nowhere to be seen, the architecture has subtly changed, and the corridors are littered with dead bodies. The Doctor informs his companions that they have arrived several thousand years early, before the solar flares that wiped Earth out, and the TARDIS will be travelling back in time to meet them.

The Ark is currently Nerva Beacon, alerting passing spacecraft that Jupiter now has a new moon - Voga, a transient asteroid that got caught in its gravitational pull. A signal seems to be coming from Voga, but someone on the station does not want this fact known. The station is also contaminated by a plague, with no cure, that kills in minutes. When the Doctor witnesses the effect of the plague for himself, he quickly makes a deduction - the plague is not a plague at all, but an alien poison.

The presence of Voga signals to the Doctor that the Cybermen are near. Voga is extremely rich in gold, a metal deadly to the Cybermen. Having lost the last Cyber wars due to the planet of gold, the Cybermen intend to demolish it so that their next attempt will be much more successful. And if the TARDIS crew stands in the way of their mission, then they too will die...