"Human Nature" is the eighth episode of the third series of the revived British science fiction television series Doctor Who. It is the first episode of a two-part story written by Paul Cornell adapted from his 1995 Doctor Who novel Human Nature. Along with its continuation, "The Family of Blood", it was nominated for the Hugo Award for Best Dramatic Presentation, Short Form in 2008.

The Doctor and Martha run to the TARDIS as an unseen enemy fires at them. The Doctor tells Martha that they are being pursued by the Family of Blood, who seek the Doctor's Time Lord life force to prevent themselves from dying. He tells Martha that he must transform into a human to escape the Family's detection until they die out, and gives her a list of instructions to follow. He uses a device called the "chameleon arch" to turn him into a human and transfer his Time Lord essence and memories into a fob watch that he asks Martha to guard.

They land on Earth in 1913. The Doctor takes the persona of John Smith, a teacher at Farringham School for boys, and Martha acts as a maid at the school. John is quiet and timid, but faint memories of the Doctor slip through in his dreams. He catalogues the dreams in a book he has titled A Journal of Impossible Things. John keeps the fob watch on his mantle, but a perception filter around it keeps him from being curious about it. John has also become infatuated with the school nurse, young widow Joan Redfern, and shares his journal with her. Martha is concerned, as the Doctor did not instruct her on what to do should he fall in love. She also struggles with her lowly position at the school, but her resentment at the arrogant public schoolboys is tempered by the knowledge that many of them will die in the upcoming World War I. Timothy Latimer, a young student at the school with ESP, discovers the fob watch and pockets it for himself.

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