After chasing an unexplained capsule through time, the TARDIS lands in London, during an air raid. Rose investigates when a small child wearing a gasmask, standing on a rooftop, continuously calls out for his mummy. After floating above the city on a barrage balloon, Rose is swept off her feet by another time traveller – Captain Jack Harkness. Meanwhile, The Doctor follows a teenaged girl to a meeting of the homeless children of the Blitz. Here, the child in the gasmask is terrorising them with a parade of strange events, constantly asking for his mother.

After narrowly escaping the gas mask monsters, The Doctor, Rose, and Captain Jack try to stop the mutated people from infecting other victims. But it soon becomes apparent that one of the group may indirectly responsible for the virus. The answer lies at the bomb site, but time is running out...