Sarah is a victim of the plague and the only cure is to transmat her down to Voga.
The Doctor escapes Kellman’s room and hears Sarah scream. He throws the Cybermat to the floor and kills it with gold dust, but Sarah has already been bitten. The Doctor carries her to the transmat chamber, handing her to Harry, and prepares to beam them down to Voga and back. However, Kellman has taken the transmat’s pentalium drive. The Doctor reconfigures the transmat to bypass the sabotaged system while Stevenson and Lester go to confront Kellman. On Voga, Vorus sees a giant rocket, the Sky Striker. He tells Magrik his agent has informed them that the Cybermen are heading for the beacon. Vorus wants the Sky Striker fitted with its bomb head in four hours.
The Doctor jury rigs the transmat, and Harry and Sarah beam down to Voga. With the poison filtered out, Sarah instantly recovers. As Harry notices that the cavern floor is littered with gold, Vogans arrive and capture them. Harry and Sarah are brought before Vorus, who wants to know who is still alive on Nerva. However, the answers will have to wait. Harry and Sarah are taken away while Vorus answers a call from Councillor Tyrum, who arranges for them to meet.
Lester and Stevenson capture Kellman. The Doctor explains that the Cybermen fear Voga because gold, a noncorrosive substance, plates their breathing apparatus and suffocates them. The Doctor cannot get Harry and Sarah back without the pentalium drive, but Kellman feigns ignorance, trying to buy time until the Cybermen arrive. The Doctor uses a control box he found in Kellman’s room to activate a Cybermat, threatening Kellman with it until he reveals that the drive is around his neck.
Harry and Sarah are chained in a cave. Harry notes that the chains are solid gold, soft metal that they may be able to file through. Meanwhile, Tyrum tells Vorus that he knows that aliens have come to Voga. He also knows that Vorus wants Voga to emerge as a trading power again and not hide from the Cybermen, who apparently disappeared centuries ago. Tyrum no longer trusts Vorus or the Guardians, and will send his militia to take over the mines. Vorus is furious, but Tyrum says his troops have orders to crush any resistance.
Fighting breaks out in the mines between the Guardians and the militia. Vorus tells Magrik to keep Tyrum from finding out about the Sky Striker, and to kill the two humans immediately. Harry and Sarah have managed to free themselves, however, and get away before the execution team arrives. They are pursued by more Guardians, who fire at them. Harry and Sarah are cornered and about to be shot when militia troops appear, forcing the Guardians to stand down.
The Doctor has repaired the transmat, but cannot lock onto Harry and Sarah as they have left the receptor circle. Lester detects an incoming ship, but it does not respond to their signals. As the Cybership docks, the Doctor recognises it for what it is, but cannot lock the hatch. The Cybermen enter, as if in a funeral march or procession. Lester and Stevenson shoot at the Cybermen but the Cybermen return fire and shoot sizzling balls of light from their head ports on their foreheads that take Lester and Stevenson down. The Doctor trys to sneak away but the Cybermen shoot him several times and he lies on the floor, looking dead. The Cyber Leader then says “All resistance overcome. The Beacon is ours!”.