The Cybermen refuse the Doctor's surrender and advance on him and the others, but the Doctor pulls the TARDIS power cell out of his pocket and fires a burst of energy that disintegrates the Cybermen. Mrs. Moore arrives in the Preachers' van and gets them out, taking Peter with them but leaving Jackie behind when they can't get to her. Jackie hides in the house as the Cybermen search the place.
The Preachers, including Rickey, blame Peter because he's been working with Lumic. In response he claims he's their inside man, Gemini. Rickey admits his reputation as "Most Wanted" is for parking tickets rather then rebellion. The Doctor has Peter remove the ear-pods and deactivate it with his sonic screwdriver in case Lumic is listening in, then tells them to go to the city and warn the authorities.
Lumic addresses the Cybermen who reveal that activation should proceed. He activates the transmission through the ear-pods, converting everyone in London wearing them into immobile puppets… including Jackie. They then march mindlessly toward the conversion centre while newscasters try to warn people. Lumic's man Crane pulls out his ear-pods rather than fall under his employer's mind control.
Peter identifies that the conversion centre is in Battersea, while Rose remembers seeing a Cyberman helmet in Van Statten's museum in "her" Earth. The Cybermen close in on them and they split up, with Rickey and Mickey going one way. They separate as well while Mrs. Moore takes the others down an alleyway where the Doctor redirects them using his sonic screwdriver.
At a conversion centre, Jackie and the others are prepared for conversion while the rejects are prepared for incineration.
Rickey ends up down an alleyway which dead ends, and he tries to climb over a gate but the Cybermen grab him and electrocute him as Mickey looks on. The Cybermen stare at him in silence and he's forced to flee.
Crane is brought before Lumic and Crane claims his ear-pod malfunctioned. He volunteers for conversion but once he gets close, he tries to sever Lumic's life support tubes. A Cybermen kills Crane and then take Lumic away for conversion.
Mickey reunites with the others and tells them what happened to Rickey. Rose embraces him but Jake is unimpressed and tells Mickey that he's nothing. The Doctor warns they will have to mourn Rickey once London is safe. They approach the Battersea conversion centre and Mrs. Moore has plans of the centre showing them a way in. Peter suggests they go in through the front door and Mrs. Moore shows them they have fake ear-pods. Peter and Rose volunteer to go in but the Doctor warns that if they show any emotion the Cybermen will see through the ruse. The Doctor realizes there's a transmitter on Lumic's zeppelin to cut off the conversion signal while the Doctor and Mrs. Moore go through the underground tunnels. Mickey insists on going with Jake and the Doctor wishes him good luck.
The Doctor and Mrs. Moore find Cybermen in the tunnels… shut down until the time is right. They slip by them while Peter and Rose infiltrate the centre. Mickey and Jake get to the top of the centre and approach the two mindless guards. Jake initially plans to kill them but Mickey convinces him not to and they use smelling salts instead to knock them out before boarding Lumic's zeppelin.
As they make their way through the tunnels, Mrs. Moore explains that she worked at Cybus until she found out what was going on, and faked her death to keep her family safe. The Cybermen notice their presence on the sensors, and activate the dormant units. The Doctor and Mrs. Moore make their way through a hatch and the Doctor seals it behind them just in time.
Rose and Peter are lead through the centre where they see the rejects incinerated while others have their flesh stripped off of them and a Cyberhelmet attached, completing the conversion process. One Cybermen recognizes Peter and identifies itself as the former Jackie Tyler and has them taken away to Cyber Control. Peter wonders if they can be converted but they lose track of her in the crowd.
After a nervous moment with a Cyberman display unit, Jake and Mickey work on disabling the transmitter controls. Below the centre, the Doctor and Mrs. Moore meet a Cyberman and Moore disables it with an electromagnetic bomb. The Doctor opens it up and examines the organic parts, and finds an emotion inhibitor. The Doctor disables the inhibitor and then apologizes when it starts feeling again and identifies itself as a woman, Sally, who was on her wedding night when she was converted. The Doctor shuts her down permanently, putting her out of her misery. The Doctor says they need to find the signal to override the inhibitor and send it to all the other units, forcing them to kill themselves. They get ready to leave but the Cybermen catch them and kill Moore. They recognize the Doctor as having two hearts and take him for analysis.
Mickey taps into the computers aboard the zeppelin, setting off a signal to the Cyberman display unit. The Doctor is taken to Peter and Rose who reveal that Jackie has been converted. Lumic enters the room, "upgraded" and seated on a throne where he is identified as the Cyber Controller.
The Cyberman aboard the zeppelin attacks Mickey and Jake, but Mickey lures it into attacking the control panel where it destroys the control signal. He and Jake tap into the security system. The humans panic and run out past the Cybermen, but Lumic is confident that his centres in other countries will assure that the Age of Steel comes to pass. The Doctor dismisses Lumic's overconfidence and boasts of his emotions. When Lumic asks him if he's known negative emotions, the Doctor admits they hurt but doesn't want to be freed from them.
The Doctor talks about how it is everyday humans that will make the difference and begins to explain how someone with appropriate computer knowledge could acquire the cancellation signal for the inhibitors. Mickey catches on and gets the signal, sends it Rose's phone, and she tosses it to the Doctor who enters it into the system. The Cybermen go into convulsions and then explode, overwhelmed by their emotions. Lumic is unaffected but the Doctor, Rose, and Peter run for the exit as the place begins to explode. They are cut off through one exit but Jake pilots the zeppelin toward the roof as Mickey tells the others to meet him up there. Lumic starts to pull free of his "throne" while the Doctor, Rose, and Peter climb up the rope ladder to the zeppelin while Mickey pulls the vehicle away from the exploding building.
Lumic manages to grab onto the rope ladder and start climbing. The Doctor gives the sonic screwdriver to Peter who is lowest on the rungs and tells him to use it. Peter cuts the ropes directly below him and Lumic falls back into the exploding building.
Later the Doctor returns to the TARDIS and puts the recharged power cell in, reactivating the TARDIS. Outside, Rose tries to explain parallel worlds to Peter and invites him in, but Peter says he must go and destroy the remaining factories. Rose calls him "Dad" but Peter refuses to hear it and goes off. Mickey and Jake come back and Mickey tells them that he's going to stay to fight the remnants of the Lumic organization and help his Gran in this universe. He tells Rose that she doesn't need him. The Doctor warns that they can't come back and gives Mickey the phone with the inhibitor code The Doctor wishes him well and then leaves Rose to exchange a final farewell before she gets into the TARDIS and well. Jake and Mickey look on as the TARDIS dematerializes.
Back in the "real" universe, the TARDIS materializes in Jackie's flat and Rose embraces her mother while the Doctor looks on. And Mickey and Jake get into a van and prepare to head out to save the world.