World War Three

Following from the cliffhanger in "Aliens of London", the Doctor's extraterrestrial constitution allows him to survive the electrical pulse administered by the Slitheen Green, which has killed the other alien experts in the room. He manages to direct the charge to Asquith, and the shock spreads to Green and the other Slitheen via their communication devices. Margaret Blaine is temporarily stunned, allowing Harriet Jones and Rose Tyler to escape the room in 10 Downing Street where she had them cornered, while Mickey Smith is able to push aside the police inspector who was advancing on Jackie Tyler. The Doctor attempts to get the police, but by the time he has returned, the Slitheen have gotten back into their suits. Green instead pins the deaths of the alien experts on the Doctor, and orders him shot on sight. The Doctor escapes to the upper floors of 10 Downing Street, and reunites with Rose and Harriet in the Cabinet Rooms where they try and determine what Emergency Protocols are in place for an alien invasion. Before sealing off the rooms, the Doctor confronts the Slitheen and learns that they are a family rather than a race, and they are not invading Earth, but rather raiding it for some commercial purpose.
The Emergency Protocols are found to be of no help as they merely list the alien experts already dead in the room downstairs. They also reveal that the United Kingdom cannot launch nuclear weapons at the Slitheen as the codes can only be released after authorization by the United Nations. Gathering all the information they have found about the Slitheen, the Doctor deduces that they are a calcium-based lifeform from Raxacoricofallapatorius, and that they use compression collars to squeeze their bodies into the human skins, which accounts for their flatulence. Although the Slitheen have shut all communications to the Cabinet Rooms, Rose's tampered phone allows her to make contact with Mickey and Jackie, now safely in the former's flat. The Doctor gives Mickey instructions on how to log on his computer into the UNIT website, and uses that to determine that the Slitheen ship is presently in the North Sea, transmitting some signal that Mickey attempts to decode. However, as he works, the Slitheen posing as a police inspector breaks in, but thanks to advice from the Doctor, Jackie kills it by dousing it with copious amounts of vinegar.
Back at Downing Street, Green and the other Slitheen declare a matter of national security and request that the UN release the activation codes to strike against a fictitious mothership that is hanging over London. The Doctor realizes that the Slitheen actually plan to fire the weapons against other countries in order to start World War III so they can sell the Earth's radioactive remains as a fuel source, which they have already begun advertising through the signal Mickey has decoded. The Doctor tells Rose and Harriet that he knows a possible solution, but that it might cost their lives. Jackie, who had been pestering him for endangering Rose's life and is still on the phone, is outraged, but Rose and Harriet both tell him to go ahead, the latter even casting her timidity aside to order him as an elected representative. Complying, the Doctor helps Mickey to hack online into the controls of the Royal Navy HMS Taurean, a Trafalgar class submarine off the coast of Plymouth, and to fire a non-nuclear Harpoon missile at 10 Downing Street, where all the Slitheen conspirators are now gathered. The area around 10 Downing is cleared while the Doctor, Rose, and Harriet take shelter in a cupboard. The Slitheen's last second attempt to escape is delayed by the difficulty of putting on their human suits and they are caught in the explosion when the missile hits. The Doctor, Rose and Harriet all survive, and Harriet takes charge of the scene with a new found authority. The Doctor - who had found her name familiar when he first met her - suddenly remembers that she is due to become a highly successful Prime Minister and "the architect of Britain's Golden Age". The entire event is dismissed in the press as a hoax, and the Doctor has Mickey use a special computer virus to wipe all records of his existence from the Internet.Jackie is now impressed by the Doctor and wants to invite him for dinner to get to know him better, but he declines fiercely. Instead, he separately invites Rose and Mickey - who has now earned his respect - to go travelling some more immediately. Mickey, still overwhelmed by the Doctor's adventurous lifestyle, declines but Rose packs some belongings and boards the Tardis.