Nerva Beacon is invaded by the Cybermen who are intent on destroying Voga.
The Cyber-Leader tells Kellman that the three men are not dead, merely neutralised; they are necessary to their plan. Kellman set the transmat receptors mere yards from a shaft that leads into the core of Voga. The environment is hostile to Cybermen. The three men will carry explosives to Voga and destroy the asteroid. Kellman insists on going down to Voga first to check that the transmat is functioning properly and the Cyber-Leader beams him down. There, he runs into some militia. Not realising the distinction between them and the Guardians, he demands to see Vorus and is taken away while trying to warn them that they are all in danger. Meanwhile, Harry and Sarah are brought before Tyrum and tell their story. When Harry mentions the cybermats, Tyrum asks Harry and Sarah to accompany him to confront Vorus.
The Doctor wonders what Kellman’s reward is, if it is not Voga’s gold. He taunts the Cyber-Leader, saying that the Cybermen were finished once humans discovered their weakness to gold and ended the Cyber-Wars. The Cyber-Leader tells the Doctor that is why Voga must be destroyed before the Cybermen resume their campaign. The Cyber-Leader says Kellman was promised the rule of the solar system after the Cybermen have conquered it.
With Cyberbombs strapped to their backs, the Doctor, Lester and Stevenson are briefed. They are to plant the bombs in the core of the planet, after which they have fourteen minutes to return and escape via transmat. If they try to remove their harnesses before they reach the target zone, a secondary explosion will kill them. Their progress will be followed by radar. They beam down, accompanied by two Cybermen. Militia arrive and start to fire on the Cybermen, who make short work of the Vogans. None of the three men believe that the Cyber-Leader will keep his word and and let them escape, but they have to keep moving towards the target zone as they are being monitored. On Nerva, the Cyber-Leader declares that Kellman is of no further use to them.
Tyrum questions Kellman, who tells him that he and Vorus were working to lure the Cybermen to the beacon, which Vorus has targeted with a rocket. At that moment, a militia man arrives to tell Tyrum about the arrival of the Cybermen, and their weapons are useless. Kellman urges them to use the rocket. Tyrum orders his men to use every weapon they can while he speaks to Vorus. Harry tells Sarah to get back to Nerva to warn the Doctor while he tries to stop the rocket from being fired.
When Tyrum tells Vorus about the Cybermen on Voga, he shows Tyrum the Sky Striker, which he has been working on for two years. With the Cybermen already on Voga, they have no time to get it ready. Vorus claims his plans were to free his people from the fear of the Cybermen and bring them back into the light. Tyrum scoffs. Vorus has allied himself with Kellman, a double agent and murderer, motivated solely by the promise of gold. Harry suggests finding another way into the core to stop the bombs.
The Cybermen continue their slaughter of the Vogans as the bomb timer ticks. Sarah transmats back to Nerva, where she overhears the Cybermen monitoring the three men’s progress. However, the deeper the three men go, the more the gold interferes with the radar. The men continue onward to the centre of the asteroid.
Harry and Kellman, meanwhile, crawl down a cross shaft towards the same place. With the exit blocked, Harry pushes against the rocks, causing a rock slide. Kellman pushes Harry out of the way, to be crushed to death by a boulder. On the other side, rocks rain down on the Doctor. Harry exits the shaft and finds the Doctor unconscious. Not knowing the danger, Harry tries to unbuckle the Doctor’s harness.