This week on Doctor Who: The Poison Sky, 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 (David Tennant) has to fight to keep both Martha (Freema Agyeman) and Donna (Catherine Tate) alive. But will he have to make the ultimate sacrifice?

Also stars Christopher Ryan as General Staal and Jaqueline King as Sylvia Noble.