Harry Saxon becomes Prime Minister and his reign of terror begins – but that's only the start of his ambitions, as he announces mankind's first contact with an alien race, the Toclafane. An audacious plan spanning the whole of time and space begins to close around the Earth, in the penultimate episode of Russell T Davies's Doctor Who.

One year has passed since the events of "The Sound of Drums." The Earth has been conquered and its population enslaved, the Doctor is the Master's prisoner, and the warships of a new Time Lord Empire rise from the ashes. The fate of the world is in Martha Jones' hands.

The Doctor takes Donna back to Pompeii, on the day of the eruption of Mt. Vesuvius, and is faced with a great moral dilemma. Should they warn the residents of the impending disaster, or let history take its natural course?