Every human in the briefing room is electrocuted -- but the Doctor isn't human, and he manages to remove his card and slap it onto the exposed Slitheen's electronic collar. The feedback incapacitates not only this Slitheen, but all others. Upstairs, Harriet and Rose bolt for the door as the Slitheen that was Margaret Blaine goes into spasm; elsewhere, Mickey bursts into Jackie's flat and pulls her to safety, pausing only to snap a picture of the writhing Slitheen with his cellphone.
The Doctor bolts out of the briefing room to fetch help, but Green manages to pull the ID card away from his fellow Slitheen's collar, and by the time the soldiers burst into the room, it has disguised itself as General Asquith again. Sergeant Price examines the experts' bodies and confirms that they're dead, and Green accuses the Doctor of killing them. The Doctor in turn accuses Green of being an alien in disguise, but the soldiers don't buy it. The Doctor tries to flee, but soldiers surround him in the corridor outside, and Asquith orders them to execute him on the spot; fortunately, they've backed him up against a lift, and he pops inside and closes the doors before they can shoot. Asquith places the upper levels of 10 Downing Street under strict quarantine and orders Price to secure the ground floor and shoot the Doctor on sight. In the meantime, Asquith and Green head upstairs -- and on the way, they dispose of their flatulent human disguises to revel in their glorious nudity.
Upstairs, Harriet realises that they've left the emergency protocols in the Cabinet office, but the Slitheen-Margaret has recovered and they are unable to return for them. The Doctor's lift stops briefly on the floor, distracting the Slitheen for a moment and enabling Harriet and Rose to hide in a nearby office. The Doctor takes the lift up one more floor and then descends via the staircase. Green and Asquith join Margaret, eager to purify their blood with a hunt; Margaret can scent the fear of her targets, and has been prolonging the hunt deliberately to make the smell more intoxicating. Margaret pulls aside the curtains to reveal Rose hiding behind them, but Harriet emerges from hiding, offering herself up in the young woman's place. Fortunately, the Doctor bursts in and drives the Slitheen back with a fire extinguisher, enabling them all to escape.
Harriet leads the Doctor back to the Cabinet office to fetch the emergency protocol booklet, but the Slitheen are only a few steps behind them. The Doctor holds them off with a bottle of port, threatening to use his sonic screwdriver to increase its flammability. When the Slitheen hesitate, he takes the opportunity to question them, and learns that they are not an invasion force, but a family business. Slitheen is not their race name, but their surname; "Joseph Green" is in fact Jocrassa Fel Fotch Pasameer-Day Slitheen. Before the Doctor can learn their intentions for the Earth, however, "Asquith" calls his bluff regarding the port. As the Slitheen advance on him, the Doctor delivers a brief lecture on the history of the Cabinet offices... up to the installation of three-inch-thick steel security bulkheads, which he now activates, sealing the Slitheen out of the room. And, unfortunately, sealing himself and his friends inside.
Satisfied that the Doctor no longer poses a threat, the Slitheen summon the rest of the family to Downing Street, and Asquith orders Price to keep the ground floor secure and await an emergency address from the Acting PM to the people of the world. Outside, Andrew Marr tries to make sense of the variety of people who are showing up: Group Captain Tennant James of the RAF; Ewan McAllister, Deputy Secretary for the Scottish Parliament; even Sylvia Dillaine, Chairman of the North Sea Boating Club. All they seem to have in common is their remarkable girth and the fact that they've been invited to the upper levels of 10 Downing Street. Upstairs, the Slitheen strip off their skin-suits, and soon, the entire family has gathered -- except for Sip Fel Fotch, "Commissioner Strickland", who has yet to finish with Jackie Tyler...
Mickey helps Jackie to slip back to his flat, unnoticed. There, he warns her that nobody can be trusted (casting a suspicious glance at his trash bin as he does so), and accuses the Doctor of dragging Rose into this dangerous lifestyle. Jackie begins to cry as the shock settles in, realising that the man she accused of killing her daughter has now saved her life -- and that the monster that tried to kill her is still out there somewhere. Outside, "Strickland" sniffs the air and dismisses the other police, telling them that he'll finish with Jackie on his own...
In the Cabinet office, the Doctor places the PM's and Ganesh's bodies in the cupboard, and Harriet sadly realises that she never knew the young secretary's name. The Doctor explains that the Slitheen's collars generate compression fields that enable them to shrink their bodies down to fit inside human skins; the gas is a side effect, and the PM's body was too slim for them to use as a disguise. It occurs to him that he knows Harriet Jones' name from somewhere, but he can't recall exactly where. Harriet reads the emergency protocol booklet but finds nothing of any help; Rose suggests that they launch a missile at the Slitheen, but due to an unfortunate incident in the UK's past, the United Nations holds the release codes for Britain's nuclear arsenal, and they can't launch a strike without UN approval. Mickey then calls Rose, and is able to reach her through the steel shielding thanks to the modifications the Doctor made to her cellphone. The Doctor swallows his pride and asks Mickey for help, giving him the secret password ("buffalo") that will enable him to log onto UNIT's secure website. As Mickey works, however, Jackie demands to know whether her daughter will be safe with the Doctor -- and the Doctor is unable to answer her question.
The UNIT website is monitoring an alien signal that the Doctor realises is being transmitted by the Slitheen. As the Doctor listens to the signal and tries to work out what it's saying, Strickland tracks Jackie down to Mickey's flat, strips off his skin-suit and begins to break down the door. Mickey offers to hold off the alien with a baseball bat while Jackie flees, but there's no other way out of the flat. For a moment, the Doctor is paralysed by indecision, but when Rose begs him to help her mother, he springs into action and starts looking for the aliens' weakness. He and his allies list all they know about the aliens, from the colour of their skin to the nature of their technology and, finally, the smell of their farts. This enables the Doctor to narrow down their planet of origin to Raxacoricofallapatorius, which means that the aliens' bodies are composed of organic calcium, weakened by the gas compression field -- which leaves them vulnerable to acetic acid. Mickey and Jackie retreat to the kitchen, where Jackie pours all of Mickey's pickled onions, gherkins, and eggs into a container and flings it at the approaching Slitheen. The Slitheen blinks in surprise for a moment and then explodes, coating Mickey's kitchen with a wet sludge. In the Cabinet rooms, Harriet recalls how Hannibal used vinegar to dissolve boulders while crossing the Alps, and she, Rose and the Doctor share a victory toast.
The death of Sip Fel Fotch hits the other Slitheen hard, and Green storms out of Downing Street to speak to the press. Mickey and Jackie watch the live broadcast on TV while the Doctor, Harriet and Rose listen over Mickey's cellphone. Outside, Green addresses the nations of the world, claiming that Earth is under attack by hostile alien forces capable of unleashing massive weapons of destruction within 45 seconds; Britain lies directly beneath the mothership, and their only hope of survival is an immediate nuclear strike. Green thus begs the UN to pass an emergency resolution and release Britain's missile codes so they can strike first and save the planet.
The Doctor, finally understanding the truth, opens the bulkheads to speak to Margaret and confirm his suspicions. The Slitheen deliberately engineered a spectacular UFO crash in order to drive the world into a panic and convince the UN to release the nuclear codes. Once the Slitheen have those codes, they'll fire Britain's nuclear arsenal at every other country on Earth, triggering World War Three. Despite the showy splash-landing, the Slitheen's ship didn't actually crash; it's still intact, and the Slitheen intend to sit out the nuclear holocaust inside. Once the planet has been reduced to a radioactive cinder, the Slitheen will sell off bits of it as fuel to every cut-price, bargain spaceship out there. The signal they're sending out is an advertisement for the sale of the century; they'll make a mint, at the cost of five billion lives. The Doctor orders the Slitheen to leave the planet immediately, or he'll stop them. Margaret and the Slitheen burst out laughing -- but the Doctor isn't laughing, and as he closes the bulkheads, Margaret begins to get a little worried.
It's midnight in New York, and the UN Security Council have convened to vote on their emergency resolution. Dawn is breaking over London, and as far as anybody knows, it could be the day humanity fights its first interplanetary war. Green and Asquith leave Price in charge of keeping the ground floor secure, while they retreat to the Prime Minister's office to wait for the phone to ring. Giggling with anticipation, the Slitheen remove their skin-suits to greet victory in their glorious nudity.
Time is running out, and the Doctor reluctantly reveals that he can stop the Slitheen -- but he can't guarantee Rose's safety. Rose is willing to let him do whatever he has to, but Jackie refuses to let him risk her daughter's life. The Doctor finally tells Jackie that this is what his life is about, standing up and making terrible decisions -- and he doesn't want to lose Rose because of it. But Harriet then points out that she's the only duly elected representative in this room, and she's thus the only one with the authority to make this decision. On behalf of the people of Britain, she orders the Doctor to carry out his plan. The Doctor thus directs Mickey over the phone, giving him the passwords he needs to hack into the Royal Navy and launch a conventional, non-nuclear missile from the nearest offshore submarine, the HMS Taurean. Jackie can't bring herself to stop Mickey, as much as he himself might want her to... and Mickey, albeit reluctantly, presses the button and fires the missile at 10 Downing Street. The Doctor, Rose and Harriet remain trapped in the Cabinet offices, and the bulkheads may not be strong enough to protect them from a direct missile strike; nevertheless, the Doctor keeps in touch with Mickey, guiding him as he blocks the counter-defences on the coast, enabling the missile to slip through. Rose and Harriet clear all of the loose objects out of the cupboard, and they and the Doctor huddle inside and wait...
Downstairs, Price receives a report of the incoming missile, and triggers the fire alarm, evacuating the building. He rushes upstairs to warn the Prime Minister of the danger -- and bursts into the office to find it filled with naked Slitheen, waiting for the red phone to ring. After an appalled pause, Price flees. Outside, the press and police scatter as the missile hurtles towards Downing Street; inside, the panic-stricken Slitheen begin fighting over their skin-suits, but they're trying to work out which disguise belongs to whom when the missile strikes, blowing 10 Downing Street to smithereens. The steel box that was the Cabinet offices tumbles end over end into the street, and the Doctor, Rose and Harriet emerge, alive and intact. When Price rushes up to investigate, Harriet immediately takes charge, ordering him to contact the UN and inform them that the crisis is over. The Doctor and Rose watch happily as Harriet addresses the press, telling them that the human race has survived the crisis undefeated. She's come a long way from the flustered backbencher she was yesterday -- and the Doctor now knows where he recognised her name. Harriet Jones is going to be a future three-term Prime Minister, the architect of Britain's forthcoming golden age.
Rose returns home for a tearful reunion with her mother, while the Doctor returns to the TARDIS and disperses the Slitheen's signal before any interested parties can pick it up. Jackie still doesn't trust the Doctor, but she's now willing to listen and learn more about Rose's new life. When the Doctor calls Rose on her cellphone and learns that Jackie has invited him to dinner, he turns the invitation down without hesitating and instead offers Rose a choice: stay home with her mother, or come with him to see things she's never seen before. When Jackie finishes making tea in the kitchen, she finds Rose packing up her belongings -- and, though she tearfully begs her daughter to stay, Rose continues packing.
Night falls over London, and the Doctor forces the young boy who spray-painted "BAD WOLF" onto the TARDIS to scrub it clean again. Mickey has bought a newspaper, and is upset to find that the alien attack is already being dismissed as a hoax; it's almost enough to make him agree with the Doctor that humans are all idiots. The Doctor concedes that Mickey's not as bad as he thought, and gives him a CD-ROM containing a virus that will erase all mention of the Doctor from the Internet, thus ensuring that nobody can get into trouble by following him. He also invites Mickey to accompany him and Rose, but Mickey admits that he can't handle the pace of the Doctor's life. He asks the Doctor not to tell Rose, however, and when Rose shows up and invites Mickey to join them, the Doctor speaks up, stating that Mickey isn't invited. Jackie, upset, pleads with Rose to stay, but Rose assures Jackie that she's just going off travelling for a while -- and, since she's travelling in a time machine, she can be back in ten seconds no matter how long she spends out there. She and the Doctor step into the TARDIS, which dematerialises as Jackie and Mickey watch. Ten seconds pass, and the TARDIS does not return. Upset, Jackie returns to her flat... while Mickey settles down with the newspaper to wait a while longer.