Arriving on Nerva Beacon
After leaving Skaro, The Doctor, Sarah Jane Smith and Harry Sullivan use the Time Ring, given to The Doctor by the Time Lords, to travel back to Nerva, but on arriving it soon becomes clear to The Doctor that they have arrived in the 29th century - a period of time many thousands of years earlier than their previous visit.
While they wait for the TARDIS to turn up – The Doctor informs Sarah and Harry that it is travelling back in time to meet them – they decide to explore and discover that the station is currently acting as a beacon warning space traffic of the existence of an asteroid that has become a new moon orbiting Jupiter. This asteroid is actually the remains of Voga, also known as the planet of gold because of the abundance of that metal that can be found there.
The Doctor, Sarah and Harry also discover that the beacon is in the grip of a fatal space plague which has killed most of the crew. The Doctor though soon realises that the 'plague' is in fact the result of poison injected into its victims by Cybermats – metallic creatures controlled by the Cybermen.
|In A Corridor Full of Dead Bodies
As well as the beacon’s crew a visiting civilian scientist, called Kellman, is also aboard. He is in fact a traitor working with a small group of Cybermen who want to destroy Voga as gold is lethal to the Cybermen as in the form of dust it can coat their chest units and so suffocate them.
After Sarah is attacked by a Cybermat, which infects her, The Doctor uses the teleport system to send her, along with Harry, to Voga so that the teleport system can remove the poison from her body. As soon as they arrive Sarah recovers and they start to explore the underground cavern they have arrived in. But it is not long before they are captured by a group of Vogans.
|The Doctor Picks a Lock
Meanwhile the Cybermen invade the beacon. Powerless to stop them The Doctor tries to get away but the Cyberleader fires its head-mounted weapon at him and he collapses to the ground. Thankfully he is only stunned. With the beacon now in the Cybermen’s control they force The Doctor and two of the remaining humans, Commander Stevenson and Lester, to take some cobalt bombs down into the heart of Voga which they then plan to detonate and so destroy the asteroid.
What the Cybermen don't know is that Kellman is really a double-agent, secretly working with one faction of the Vogan race - The Guardians led by Vorus. Their plan is to lure the Cybermen onto the beacon and then destroy it with a rocket, known as the Skystriker. After being chained up with chains made of solid gold - which is a soft metal so making them easier to file through - Sarah and Harry manage to escape. On learning of the Vogans plan Sarah returns to the beacon, so as to warn The Doctor, unaware that he is no longer aboard.
Back on Voga, deep underground, Harry, who has become aware of The Doctor’s presence on Voga, locates The Doctor but, after causing a rockslide, finds him unconscious, After concluding that there is nothing seriously wrong with The Doctor, Harry starts to unbuckle the straps by means of which the Cybermen's bomb is attached to The Doctor's back, unaware that this will cause it to detonate. Luckily they are so far underground that the signal, to warn the Cybermen that the bomb has been tampered with, does not penetrate through the dense rock. The Doctor soon recovers and prevents the other bombs from going off.
When The Doctor enquires about Sarah, Harry informs The Doctor that she had earlier returned to the beacon to warn him about the Vogan’s plans to destroy the beacon with the Cybermen aboard. The Doctor therefore rushes back to the beacon so as to rescue Sarah. But when he arrives, by the teleport system, he discovers that she has been captured by the Cybermen. In trying to rescue her he also gets captured himself.
Frustrated at their plans being thwarted by The Doctor, and on learning of the Vogans' intentions, the Cybermen evacuate the beacon leaving The Doctor and Sarah tied up. But not before they load even more bombs onto the beacon and then set it on a collision course for Voga. At the same time the Vogan's launch their missile at the beacon.
With the missile heading towards them The Doctor manages to free himself and gives instructions for it to be redirected away from the beacon and onto a collision course with the Cybermen's ship, which is thus destroyed. This just leaves The Doctor with the task of preventing the out of control beacon from colliding with Voga which he manages to do by some nifty piloting.
Harry then teleports back to the beacon just as the TARDIS conveniently arrives on the beacon, having been gradually drifting back to this point in time. The Doctor, Sarah and Harry then depart for a rendezvous with The Brigadier, who has sent an urgent message requesting assistance with a problem on Earth.