The Poison Sky

As the poisonous gas from the Tenth Sontaran Battle Fleet's ATMOS continues to thicken throughout the world, purging humanity one country after the next, a clone of Martha Jones works in the shadows to support them. Will the Tenth Doctor learn about Martha's replacement? And can he figure out what the gas is made of, and destroy it before the entire planet chokes to death? More importantly, can the Doctor stop UNIT from starting an interplanetary war with the deadly Sontarans? It's all down to Donna Noble and child genius Luke Rattigan to fix things before it's too late.