The Empty Child (1) (S01E09)

In pursuit of a mysterious capsule through time, the TARDIS materializes in London amid an air raid. Rose discovers a young boy clad in a gas mask, standing on a rooftop and repeatedly calling for his mother. While she floats over the city on a barrage balloon, she encounters another time traveler, Captain Jack Harkness, who sweeps her off her feet. Meanwhile, The Doctor tracks a teenage girl to a gathering of homeless children during the Blitz, where the gas-masked boy instills fear among them with bizarre occurrences, persistently searching for his mum.