The MegaTech Centre in London is Britain’s biggest computer and electronics store. At 3am two silver giants materialize in the vast showroom and murder both of the security guards. They move through the store and steal some of the equipment. The next day’s police statement talks of a ‘well organized robbery by a well-equipped and professional gang’. The mysteries of the alarm system’s failure to respond and the exact cause of death of the two men remain unsolved. The second, younger, guard lived long enough to speak of ‘silver giants’.
A secret research lab in the centre of Whitehall behind the Ministry of Science, a high security building, is also attacked. Most of the night staff is killed but the sole survivor also talks about giant silver figures.
A cold, damp night at Chadwick Green Government Research and Development Centre is also disturbed. A sentry spots a silver figure within the gates. He challenges the figure who fires an energy pulse at him, narrowly missing. The sentry fires shot after shot at the figure with no effect until a second blast from the giant kills him. The alarm is raised as a second giant materializes but the emerging soldiers are ruthlessly killed by the invaders and an armoured car is destroyed. The giants smash their way into a laboratory.
Martha and the Doctor are in a prehistoric jungle observing at close hand a feeding Apatosaurus when they are attacked by a Tyrannosaurus. They flee to the TARDIS which fades away as its jaws close on the space where it stood. Immediately a square shape shimmers into existence where the TARDIS had been.
On board the TARDIS Martha asks to be taken to the Royal Hope Hospital where she had been working when she met the Doctor.
The army rebuilds Chadwick Green and replaces the dead soldiers and scientists, covering up the deaths with compensation payouts and fictional reports of scientific accidents. Major Tom Burton, a massive man in charge of security and his severely beautiful number two, Captain Sheila Sarandon, watch footage of the attack on the MegaTech Centre and conclude that the Cybermen are back. They wonder if another invasion is imminent but wonder why only two Cybermen are seen at any time. Despite their efforts on an information embargo rumours are starting on the Internet.
Burton leafs through a file on the Doctor and decides to ask the police to look out for an old blue police box.
The Doctor is puzzled by unusual signals showing matter transmission in London and suspects someone of using alien technology. Possibly aliens. He reassures Martha that it cannot be Cybermen as all of them were consigned to the Void. He lands the TARDIS in the hospital car park, and sets off to scout around while Martha heads into the hospital.
PC Jim Wilkie spots the TARDIS and calls his sighting in.
In a shadowy dome three Cybermen are surrounded by a mixture of technology, some of it their own, some stolen. They have set up a force field and a monitor to detect the Doctor. They need him to help them survive because they do not have the know-how to re-open the Void. Their monitor suddenly bleeps and tells them that the Doctor is back in London.
Inside Nick’s Internet Café the Doctor rapidly scans a number of news channels and finds out about the robberies at the computer store and government lab as well as the attack on an army base. The reports are vague but he surmises the use of teleportation. He checks a conspiracy website which concludes that the Cybermen have returned.
Martha chats with her friend Rachel in the hospital before heading back to the car park. Outside the TARDIS she is surprised by the sudden appearance of a Cyberman. It demands to know where the Doctor is.
The Doctor promptly steps out from behind a van and makes a run past the Cyberman to the TARDIS when a second silver giant materializes and grabs him. However, both Cybermen vanish immediately.
The Doctor and Martha go to the hospital canteen for a cup of tea. The Doctor thinks the sudden disappearance of the Cybermen was due to a malfunction in their transportation device. He has never known them to teleport before and suspects they are using alien technology. He also guesses that these Cybermen were created on Earth prior to the attack on Canary Wharf and were thus not sucked into the Void.
Their conversation is interrupted by the arrival of Captain Sarandon who arrests the Doctor. Martha manages to blend into the crowd while the Doctor is taken away. He is driven off in an army staff car while the TARDIS follows on the back of a truck.
In the domed building the Cybermen argue over the failure of their equipment and the Cyberleader decides to use the now-repaired device to transport back to the car park.
In the staff car the Doctor’s attempts at conversation are halted by a jab in the ribs from a military policeman. Once they arrive at Chadwick Green Camp the Doctor pulls this man’s nose to enrage him, and when he is attacked steps aside and relieves the MP of his revolver. He then hands this to Captain Sarandon and heads off to see Major Burton, demanding tea and sandwiches of his escort.
Martha returns to the space where the TARDIS had been and is grabbed by the Cyberleader. PC Jim Wilkie tries to help but the giant and the girl both vanish.
The Doctor tells Burton that he knows the base was attacked by Cybermen, demands to be given a free hand in the mission to stop them and asks for Martha to be brought down to meet him. He tells Burton of his guess that there are only a handful of Cybermen and that their plan is to catch him so that he will open up the Void.
Martha realizes she has been taken to the Millennium Dome where the Cybermen have their base. They threaten to kill her unless she says where the Doctor is but when she refuses they decide to use her as a hostage for when he turns up.
Meanwhile the Doctor hears the report of her capture and phones her mobile. He tells her immediately that he is at Chadwick Green but she can only say she is at “the Do-“ before her phone is smashed by a Cyberman. It is enough, however, for the Doctor to work out where she is. The Cyberman, similarly, heard the Doctor’s whereabouts.
In the Dome five more Cybermen are roused from hibernation. They and the Cyberleader transport to Chadwick Green.
Burton sends an assault force off to the Dome. Sarandon asks why the Doctor told the Cybermen where he was but he only tells her it seemed like a good idea.
Immediately, the six Cybermen are seen coming across the parade ground. Bullets are useless against them. Burton orders two men forward with a new and experimental rocket launcher. The first shell destroys a Cyberman but the soldiers are killed when the other silver giants shoot the launcher. A second team of soldiers appear with another launcher and blow up two Cybermen before they, too, are killed and their weapon is damaged. A final launcher is used to explode a fourth Cyberman before the soldiers are killed but the launcher remains intact. Burton and the Doctor use it to take down the fifth Cyberman but he simultaneously shoots at them, killing Burton. The Doctor is left facing the Cyberleader.
The Leader tells the Doctor to help him or Martha will die and then vanishes. Sarandon decides to join the assault force at the Dome but the Doctor says that if she gives him the TARDIS he can end things with no further loss of life. She says that the assault will go on but he can have the TARDIS anyway.
The Cyberleader returns to the Dome and announces that the Doctor is on his way. Once the Void is open he will be killed. The TARDIS arrives soon after.
The Doctor looks over the Cybermen’s equipment and tells them that he can use their teleport to reach into the Void but he needs to power it from the TARDIS.
As he does so the assault force arrives outside and finds their way barred by the force field. The Doctor powers up the teleport and a rectangular hole appears in the Dome. He tells them this is a portal into the Void. While the Cybermen are distracted Martha attacks the force field generator. A Cyberman shoots at her but misses, destroying the generator.
A giant T-Rex emerges from the portal and rips off a Cyberman’s head. Another Cyberman shoots at the dinosaur which grabs him in its jaws and drags him back through the portal. The Doctor disconnects the power cable and the portal vanishes, and then thrusts the cable into the chest of the Cyberleader, destroying him.
The Doctor tells Martha that he created a space-time portal to the exact moment and place from which the TARDIS vanished as the T-Rex lunged at it.
Sheila Sarandon runs up in time for the Doctor to point to the corpse of the Cyberleader, the last Cyberman on Earth. The time travelers step into the TARDIS and dematerialize.