The episode includes several references to outside the show's fictional universe: the episode is dedicated to Verity Lambert, "Doctor Who"s founding producer, who died on 22 November 2007, a day before the show's forty-fourth anniversary; and the malfunctioning Host stuttering over the name "Max" is a reference to 1980s virtual presenter Max Headroom; Davies inserted references to other "Doctor Who" episodes in the script: he emphasised society's increasing awareness of aliens and the tradition of London's consecutive Christmas attacks in the script, describing the latter as "becoming a bit of an in-joke"; the Doctor's use of the catchphrase ""allons-y" Alonso" in the episode when he helps Frame stabilise the ship continues a running gag originating in "Army of Ghosts"; and the Host continue the thematic motif of angels. Angels previously appeared in "Blink", where the antagonists of the episode were Weeping Angels, and in "The Sound of Drums" and "Last of the Time Lords", where the Master's communication network was called the "Archangel Network". Despite angels being the antagonist in two episodes that aired close to each other, which dismayed writer and executive producer Russell T Davies when he read Steven Moffat's script for "Blink", the Host are functionally different as subordinate "robot butlers".