Cutler ignores the Doctor's warnings that they will get visitors and continues to work to get the Zeus spacecraft to return. International Space Command tries to reach them but can't.
Outside, the metal men conceal the soldiers' bodies and don their parkas to sneak into the base.
As Barclay prepares to bring the spacecraft back, the aliens enter the building. A soldier attacks them but is gunned down with an energy weapon. The leader identifies them as Cybermen from the planet Mondas. They took the planet to the edge of space but have now returned. They were once human but replaced their bodies with cybernetic parts. Their replacement has included replacing their brains, which has removed all emotion. Cutler alerts Space Command that they are in danger. The leader orders Cutler to call off the emergency. When he refuses, they knock him out and force Barclay to respond by threatening to destroy the communications equipment. He tells them the Zeus 4 is doomed due to Mondas' gravity.
Ben wants to fight the Cybermen and grabs a gun. But the Cybermen catch him and destroy the gun with their bare hands before taking Ben prisoner and putting him in a film room.
The Zeus 4 astronauts try to return but burn up their entire fuel supply without any success. As the Cybermen predicted, they are doomed. The polar base finally loses contact with the ship. It must have exploded.
The Cyberman tells the humans that Mondas is almost exhausted of energy and has returned to drain all of Earth's energy. He offers to save the humans by taking them back to Mondas and turning them into Cybermen.
Ben turns the projector toward the door and calls in the Cyberman. With it blinded, he pulls off its weapon. The Cyberman attacks him and Ben guns him down. He sneaks back into the control room and hands the gun to Cutler, who quickly blasts down the other two Cybermen. He then contacts Geneva and warns them of the danger. Wigner tells them that they sent up a solo astronaut to try to help Zeus 4 and Cutler needs to guide him back. The astronaut is Cutler's son.
Cutler seals up the base and the Earth prepares for its first interplanetary war. They try to contact the space capsule. Suddenly, signals appears on the early warning system: hundreds of spaceships flying in formation from Mondas…