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...
The plot device of nanogenes in the episode The Doctor Dances points up the role of futuristic science concepts in Doctor Who. This episode examines some of these concepts.
