Back on Nerva Beacon, the Doctor, Sarah and Harry discover human corpses sealed off from the rest of the station. One of the survivors, Kellman, is a scientist investigating Voga, a planetoid recently caught in orbit around Jupiter. The Commander tells of a mysterious plague which killed most of the crew. By the time Sarah is attacked by th plague, the Doctor knows the cause: Cybermats controlled by the Cybermen. With the Doctor fixing the sabotaged transmat, Harry and Sarah are transmatted to Voga. This cures Sarah, but she and Harry are captured by the Vorus, a Vogan Guardian who has been plotting with Kellman to lead the Cybermen into a trap on Nerva Beacon. Another Vogan suspects Vorus of treachery, and civil war breaks out between the two factions. The Cybermen arrive on Nerva Beacon and then on Voga. The Doctor and two surviving crewmen are fitted with booby-trapped Cyberbombs and are sent to the center of Voga. Meanwhile, Vorus aims his rocket at Nerva Beacon in a hope that it will hit the Beacon before the Cybermen leave.