Synopsis: The Doctor takes Rose to London in the year 1966, where they find beautiful girls wandering the streets, doing good on behalf of a group called Lend-a-Hand. The Doctor notes that the girls don't smell human, and becomes concerned when he discovers that someone is preparing to build affordable family housing on the fields where Rose's council estate is supposed to be built. The Doctor visits a pub to check the newspaper and find out if anything else has gone wrong with history, and while there, he smells something odd from the women's toilets and investigates to find that a Lend-a-Hand girl has just disintegrated into mush. The Doctor follows a woman who had emerged from the toilets moments before, while Rose visits Lend-a-Hand House to investigate. There, she and the dumpy Shirley Gilbert are allowed into the interview room by Mr Love, only to be confronted by an alien that tries to scan their minds. Rose and Shirley panic and flee, but the alien has seen an image of the Doctor in Rose's mind, and it orders its girls to keep an eye out for the stranger, fearing that he may be the one behind the recent attacks on Lend-a-Hand girls.
The Doctor has followed the real killer to St Merrion's University Hospital, where he discovers that she is Charlotte Cobb, a scientist studying the effects of new perfumes and colognes on skin tissue. The Lend-a-Hand girls attack them both, but they flee to safety and are reunited with Rose and Shirley back at the pub. There, Charlotte explains that the Lend-a-Hand girls have killed a number of prominent scientists, including her husband, Peter, who was conducting research into DNA resequencing. The Doctor sends Rose and Charlotte to look into Peter's research and find out what made him a target; in the meantime, the Doctor will investigate Lend-a-Hand house. Shirley follows him, unwilling to be left out, and together, they find a room containing organic alien technology -- including cloning vats that are growing an army of Lend-a-Hand girls.
The Doctor and Shirley are then captured by Mr Love, himself an artificial organic construct, and the alien, a Kustollon named Igrix. Humanity and the Kustollons are due to engage in a terrible war in the year 3045, and both species will be devastated by the conflict. However, Igrix has used a stolen time machine to travel back to the 1960s in order to make humanity content with its lot and ensure that the human race never goes into space to encounter the Kustollons in the first place. Sadly, his plan has involved killing scientists whose inventions and discoveries would have pandered to mankind's aggressive tendencies; however, for the most part, he claims that he wants to make humanity happier. He thus intends to destroy the Moon so humanity will no longer have a convenient stepping stone for its space programme; then, having built up Lend-a-Hand's reputation with his clones and the real girls he hired to keep the numbers up, he will unleash his army of Lend-a-Hand girls upon the populace to improve things by using their low-level telepathy to find out what people desire and provide it to them (such as affordable housing).
Igrix gives Shirley a pill that turns her into a vacuous, smiling doll like the other Lend-a-Hand girls, but the Doctor gives him the slip and returns to St Merrion's. There, he realises that Peter Cobb was killed because his DNA research, combined with Charlotte's work in perfume, could have weapons applications. The Doctor combines Peter's and Charlotte's work and returns to Lend-a-Hand, where Igrix is about to unleash the Lend-a-Hand girls upon the streets; however, when Rose throws a bottle of perfume into their midst, the girls suddenly become unhappy and confused. The Doctor has used the perfume to carry an airborne virus that rewrites the girls' genetic coding, turning them into ordinary human beings, while the non-sentient technology simply dissolves. Igrix flees back to his spaceship atop the Post Office Tower, intending to destroy the Moon as he had planned, but the Doctor and Rose follow him there. Unable to see the moon through the cloud cover, Igrix boards his ship and launches into the air, only to find that the Doctor has infected his ship with the 'human virus.' Unsure of what it really wants to do with its life, the ship sets off to explore the Universe, ignoring Igrix's furious ranting. Before leaving, the Doctor pops by the construction site and uses his sonic screwdriver to demolish the work that's already been done, ensuring that the council flats will be built on schedule.