The Doctor manages to remove his electrified sabotage badge, explaining that it's deadly to humans but he isn't human. He pushes it into the General Asquith Slitheen, electrifying him and the Joseph Green Slitheen in his human skin suit. The backlash also affects the Strickland Slitheen attacking Jackie, and the Margaret Blaine Slitheen advancing on Rose and Harriet Jones. The Doctor runs out of the conference room, while Rose and Harriet slip past their Slitheen.
Mickey hears the commotion and runs into Jackie's flat, knocks the Slitheen over the head, and captures a photo of it on his cell phone.
The Doctor notifies the security forces that there are aliens inside of Downing Street.
Joseph manages to remove the badge from Asquith, deactivating the charge. He tells him to get into his skin suit before the security forces arrive.
Harriet realizes they left the emergency protocols behind. A Slitheen finds them and gives chase.
The Doctor brings the security forces back, but Joseph and Asquith claim that the Doctor killed the experts. The military police believe him and the Doctor runs away only to quickly be cornered and captured. Asquith orders the soldiers to kill him, however, the Doctor backs into the lift and sabotages the controls. At the next floor, he finds Margaret outside. He closes the door, unaware that the two women are nearby. Meanwhile, Rose and Harriet return to the conference room and hide.
Joseph and Asquith order the soldiers to secure the upper floors and shoot the Doctor on sight, and refuse any escort. Once they're alone, the two Slitheen remove their skin suits and rejoice in their freedom.
The Margaret Slitheen enters the conference room and closes in on the women. Joseph and Asquith come in to join in the hunt and find Rose. Harriet tries to draw their attention, and the Doctor comes in with a fire extinguisher and drives them back.
The trio runs back through the hallways and circle to the conference room. The Doctor grabs a bottle of port and bluffs, claiming he can explode it if the Slitheen attack him. They hesitate and he demands to know what their plan is. They explain that the Slitheen is their family name, not their species, and they are there to make a profit. However, they then realize the Doctor is bluffing, and that he's not human. The Doctor explains that the Cabinet Room can be sealed, and then closes the shutters, locking the Slitheen out. However, Rose points out that they can't get out.
The Slitheen cut off communications to the room and then summon the rest of their family.
Mickey takes Jackie past the police force to his apartment.
The disguised Slitheen arrive at 10 Downing Street and enter past the press. Margaret, back in her skin suit, greets them. Asquith informs his soldiers they are to stay off of the top floors and that the acting Prime Minister will soon be making a speech to the world.
In his apartment, Mickey warns Jackie that danger follows the Doctor wherever he goes, but he's their best hope of defeating the Slitheen. When she breaks into tears, Mickey tries to reassure her that no one will look for her there since she hates him. Outside, Strickland is back in his skin suit and assures the human police he plans to finish with Jackie quite soon.
All of the Slitheen except for Strickland have gathered at 10 Downing Street. In the conference room, the Doctor moves Ganesh's body out of the way and Harriet admits she never learned his name. The Doctor explains the Slitheen have to fit into larger human skin suits, and the device on their necks shrinks them down. He thinks that Harriet Jones name sounds familiar, but can't place it. She informs them that the emergency protocols for nuclear weapons are redundant and the codes are held by the UN. They can't be used without UN resolution. The Doctor tries to figure out how the Slitheen family plan to profit and where he's heard Harriet's name.
Mickey calls Rose on the Doctor's modified cell phone, and sends a photo of the Slitheen, and assures Rose that Jackie is okay. The Doctor has Mickey access the UNIT web site, while Jackie asks the Doctor if Rose will always be safe with him. Before the Doctor can answer, Mickey gets into the web site and tracks the Slitheen's ship signal. While Mickey works, the doorbell rings. Jackie goes to get it and discovers Strickland. Mickey grabs a bat and plans to hold it off while Jackie runs. The Doctor tries to identify what planet the Slitheen come from, going over all the facts they know so they can find a weakness. He realizes they are from Raxacoricofallapatorius and are creatures of living calcium. He tells Mickey and Jackie to get into the kitchen and find vinegar. The Slitheen breaks through the doors and Jackie throws a pitcher of vinegar on it, and it violently explodes.
Asquith informs Joseph that the Strickland Slitheen is dead, since they can sense when one of their own dies. Joseph then goes out and claims that they have discovered aliens in orbit above them. He asks the UN for permission to give them access to their nuclear weapons to fire at the alleged invaders. The Doctor realizes that the Slitheen hope the humans will panic and release the codes. He opens the doors and confronts Margaret, and explains his conclusion that they plan to start World War Three. Margaret confirms his theory and explains they are sending an advertisement into space to attract buyers. The Slitheen will sell off Earth's radioactive as fuel and make a profit, and escape in their crashed ship which is only parked. The Doctor tells her to leave now or he'll stop her and her family, but Margaret laughs at his threat and he reseals the door.
The next morning, everyone hides in their houses and waits for the UN resolution. Joseph and the other Slitheen go to the Prime Minister's office and wait for the call from the UN.
With time running out, the Doctor finally says that there has always been a way out. However, he warns Jackie that if he uses it, there's no guarantee that Rose will survive. Rose tells him to do it without knowing what it is, despite Jackie's pleas. When the Doctor hesitates, Harriet Jones steps forward as a representative of the people and tells him to do it.
In the Prime Minister's office, the Slitheen remove their skin suits to celebrate their coming victory in their true form.
The Doctor gives Mickey the password to hack into the Royal Navy database. He locates a sub with missiles and Jackie threatens to stop him. Mickey tells her to do it if she has to, and she backs off. The Doctor tells him to fire and a missile launches from the North Sea and heads for 10 Downing Street. Rose suggests they take refuge in a shelter. Meanwhile, the UN votes to release the nuclear code. The Slitheen, gathered in the office, get word.
With the Doctor's help, Mickey neutralizes the anti-missile defenses. The soldiers get word that the missile is coming and sound the alert, and barges into the office. He quickly runs out after announcing there's a missile on the way. The Slitheen start fighting among themselves while the humans below evacuate the area. The Doctor, Rose, and Harriet take shelter in the closet as the missile hits the building. The trio emerge from their improvised shelter and Harriet takes command, telling the soldiers to notify the UN that the crisis is over. The Doctor suggests she should take command as Prime Minister but Harriet insists she's just a back bencher. As they go, the Doctor explains to Rose that Harriet is the future architect of Britain's Golden Age and will serve for three terms.
Rose returns to the flat and embraces Jackie. Later, they watch TV and Jackie reluctantly admits that the Doctor helped save the world. She accuses Rose of being infatuated with the Doctor but Rose denies it, and Jackie wants to make dinner so she can learn more about the Doctor. The Doctor calls from the TARDIS to tell Rose he'll be ready to go in a couple of hours. Rose invites him to dinner, but he insists he doesn't do domestic. The Doctor describes his next trip to a plasma wave, and tells Rose to choose that or dinner with her mother. When Jackie comes back, she discovers that Rose is packing to leave. She asks her daughter not to leave.
The Doctor finds Mickey waiting for him outside the TARDIS. Mickey explains that the newspapers are dismissing the Slitheen as a hoax. The Doctor gives Mickey a virus that will destroy any mention of the Doctor, and admits that Mickey is right and he's dangerous. He invites Mickey to travel with them and look after Rose, but he admits that he can't handle that kind of constant adventure. Rose and Jackie arrive, and Rose assures her that she'll be back. She invites Mickey to join them, and when he hesitates, the Doctor covers for him and says he forbids Mickey from coming with them. Jackie challenges the Doctor, wanting a guarantee for Rose, but Rose insists they'll be back in a few seconds of her mother's time. The Doctor and Rose leave in the TARDIS as Mickey and Jackie wait ten seconds... and more.