About Doctor Who Season 4, Episode 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... Guest Stars: Billie Piper, Freema Agyeman, Christopher Ryan, Ryan Sampson, Christian Cooke, Dan Starkey, Bridget Hodgson, Rupert Holliday-Evans, Leeshon Alexander, Wesley Theobald, Kirsty Wark, Bernard Cribbins, Clive Standen, Meryl Fernandes, Lachele Carl, Jacqueline King,