A terrifying alien army is sweeping across the landscape, decimating towns and subjugating everyone and everything in its path. With their astute military tactics and advanced weaponry, the invaders seem unstoppable. But this is no distant star, no alternate timeline. Trapped in a frightened city, the Doctor and his companions discover that this is Earth history, and they are powerless to intervene. The impending slaughter of thousands is a matter of grim historical fact. For, deep beneath the city, something truly alien is stirring...