The Sontarans activate their masterplan and begin to choke the whole of planet Earth, as Russell T Davies's Bafta Award-winning time-travelling drama continues. UNIT is left defenceless, with a traitor in their ranks. And as interplanetary war edges closer, the Doctor has to fight to keep both Martha and Donna alive – but will he have to make the ultimate sacrifice?

David Tennant plays the Doctor and Catherine Tate plays his companion, Donna Noble. Freema Agyeman and Christopher Ryan guest star.