S01E10: 'The Doctor Dances'
With the help of some of Jack's gadgets, they find the eerie Room 802 - a cell where the mysterious child they encountered earlier was kept. They discover an audio recording of the young boy's pleas for his mother, along with scores of drawings on the wall. But the boy soon pays them a visit and they quickly head off to avoid infection.
Nancy breaks through the hospital's perimeter fences but is caught and handcuffed to a desk, guarded by a soldier with a suspicious mark on the back of his hand - just like the one Dr Constantine had before he turned into a gasmask zombie. The plague quickly takes hold of the young man and his face changes, but Nancy sings him a lullaby and the soldier falls asleep.
Jack, who earlier made a discreet exit from the hospital, teleports The Doctor and Rose to the safety of his own ship. Whilst onboard, The Doctor finds out about the presence of nanobots in Jack's ship - microscopic robots that were earlier used to heal Rose's rope burns. This leads him to discover that the ambulance ship that led him to 1941 London is full of the nanobots, who are trying to heal victims using the model of a gasmask wearing child who seemingly perished in a bomb explosion.
Finding the cause is one thing, but working out the solution before the gasmask-wearing zombies convert mankind into their own image is far trickier. Especially when the answer lies with a young woman afraid to face up to her past...