As with the previous episode, scriptwriter Paul Cornell based this episode upon his 1995 Doctor Who novel, "Human Nature", making this two-parter the first time in series history that an original Doctor Who novel was adapted for television.
Doctor Who as John Smith mentions that his parents were called Verity and Sydney. This is a tribute to Verity Lambert, the first producer of the original series "Doctor Who" (1963), and Sydney Newman, the Head of Drama at the BBC who gave her this job.