As the ships approach, the Doctor collapses. Ben and a soldier take him to a back room. Cutler raises the Zeus 5, which is experiencing power loss when it's near Mondas. He doesn't tell his son what happened to Zeus 4 but concentrates on bringing down Zeus 5.
Cutler decides they must destroy Mondas to stop the invasion. They will use a Z-bomb, a doomsday weapon, to destroy the planet. Wigner wants to wait because of the danger to Earth but Cutler is eager to act and gets Wigner to agree to any action necessary to stop the Cybermen. Ben objects, saying the Doctor had said that Mondas would absorb too much energy and destroy itself. They should wait. But Cutler is determined to strike, hitting Mondas when his son is on the far side of Earth. Ben is taken away but he asks Polly to work on Barclay to stop Cutler.
Cutler and Dyson begin working on the Z-bomb. Dyson is nervous about using the weapon, but Cutler is determined. Polly draw out Barclay on the dangers of the Z-bomb. She suggests that they quietly sabotage the bomb so that they appear to be ready to launch but that it doesn't work.
A ship is spotted coming to the polar base. Rather than shoot it down, Cutler has his men let the ship land and lay an ambush.
Polly takes Barclay to see Ben. He comes up with an idea to sabotage the rocket in a way that can't be traced. Ben will go through the ventilation ducts and Barclay will distract the guard and technician, giving Ben a chance.
The Cybership lands and more Cybermen emerge. They are easily gunned down by Cutler's men.
Ben crawls through the ducts and reaches the silo.
Cutler has his men take the Cyber weapons. When he can't find Barclay, he heads out to the silo and finds Ben sabotaging the system. He throws Ben over a railing and has him taken prisoner. Cutler says that if the rocket fails, he will take the law into his own hands with Ben and Barclay.
The base crew begin the countdown to the Z-bomb launch. Ben recovers but, due to the injury from the fight, can't remember if he actually sabotaged the rocket. Everything appears to go smooth as the rocket is moved to launch position. The countdown gets to zero and the rocket's engines fire…