The original Doctor Who (1963) presented conflicting evidence about when the UNIT stories were meant to take place, and there has been much confusion and continuing fan debate on this subject. The production team never reached consensus as to whether the UNIT stories took place in the "present" (1960s-70s) or the "near future" (1970s-80s). The Doctor references this confusion when he says he was on the UNIT staff "in the 1970s, or was it the 80s?"
This story marks the first appearance of the Sontarans since The Two Doctors: Part Three (1985), 23 years earlier.
Originally, Geoff Noble was in this episode. Helen Raynor drew upon her own parents' relationship, depicting Geoff being henpecked by Sylvia. However, after filming his scenes for Partners in Crime (2008), it had become clear that Howard Attfield was extremely ill with cancer. After consulting with the actor's wife, Russell T. Davies decided to replace him with Wilf. One draft explained that Geoff was unwell.
Initially, cloning was going to be a much more persistent element of the story, with the ATMOS factory workers all being clones. Russell T. Davies felt that the clone idea would be more effective in small doses, however, which led to the introduction of the cloned Martha.