The TARDIS brings the Doctor to Earth, where he almost immediately runs into Donna Noble, her husband Shaun, and their daughter Rose -- just before a spaceship comes down in the middle of London. Desperate to avoid triggering the metacrisis which will inevitably cause Donna's demise, the Doctor tracks the vessel to a steelworks, where the responding UNIT soldiers are behaving very strangely. Meanwhile, an impish alien called the Meep has escaped the ship and found shelter with Rose. When the Wrarth Warriors invade the Noble residence, the Doctor may have no choice but to imperil the life of one of his best friends.