"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.