Doctor Who: Season 4, Episode 5 - ProjectFreeTV

4 - 5: The Poison Sky

The Sontaran fleet watch from space as the Earth is gripped in a deadly blanket of poisonous fumes. Whilst UNIT collides with Sontaran ground forces, the Doctor and a treacherous clone of Martha attempt to locate a missing TARDIS, and Donna finds herself trapped aboard the Sontaran ship itself...