The Poison Sky

"The Poison Sky" is the fifth episode of the fourth series of British science fiction television series Doctor Who. It was broadcast on BBC One on 3 May 2008. The episode features both former companion Martha Jones and the alien Sontarans. It is the second of a two-part story; the first part, "The Sontaran Stratagem", was broadcast on 26 April. Continuing from the previous episode, "The Poison Sky" reveals the poisonous gas that is choking the Earth's atmosphere is intended as a way of feeding new batches of Sontaran clones that would be bred on Earth for war. Plot Four hundred million cars on Earth equipped with ATMOS devices are suffocating humanity with poisonous gas. Sylvia frees Wilfred from her car by breaking the windscreen. The Tenth Doctor tells Donna's family to get inside and seal the windows and doors as best they can. He and Donna return to the ATMOS factory, where the Doctor warns UNIT not to engage the Sontarans. The Doctor tells Donna to stay in the TARDIS for her own safety, but the Sontarans locate and teleport the TARDIS aboard their ship. When the Sontarans prevent UNIT from launching a nuclear missile at their ship with help from the clone of Martha, the Doctor works out that since their ship was safe, the Sontarans were really preventing the disruption of their atmospheric conversion. The Sontarans attack and take over the factory, easily overwhelming UNIT troops. UNIT manages a counterattack and calls in the aircraft carrier Valiant, which puts the Sontarans on the defensive. Following UNIT's offensive, the Doctor ventures into the factory and discovers the real Martha in one of the Sontarans' cloning devices. Having long suspected the truth, the Doctor awakens the real Martha, killing the clone in the process. Before dying, the clone reveals the gas is clone feed, which the Doctor deduces is used to breed billions of Sontaran soldiers on Earth. The Doctor tells Martha to keep UNIT from launching any missiles and rushes off.