It's 1913 in England, and an ordinary schoolteacher called John Smith dreams of adventures in time and space and a mysterious blue box, as Russell T Davies's Doctor Who continues. But, when lights in the sky herald the arrival of something strange and terrible, Smith's maid, Martha, has to convince him that he alone can save the world. David Tennant plays the Doctor and Freema Agyeman plays his companion, Martha Jones. Jessica Stevenson and Thomas Sangster guest star. Human Nature will air on Saturday 26th May at 7pm on BBC One.