S4, E5: The Sontarans activate their masterplan and begin to choke the whole of planet Earth. 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?
Starring Freema Agyeman, Billie Piper, Biddy Hodson