Professor Yana's costume was inspired by the First Doctor's outfit. Russell T. Davies even told the costume designer to "Hartnell him up".
Upon first meeting, the Doctor ask Jack if he's "had work done". John Barrowman, by his own admission, had some minor botox done around his eyes, probably during the year and a half between his appearances on this series.
This is the first time in the revived series that The Doctor uses his sonic screwdriver in a vertical fashion, reminiscent of Doctor Who (1963)'s old sonic screwdrivers.
Nine year old John Bell from Kilmacolm, Renfrewshire, Scotland, entered a Blue Peter (1958) competition to appear in this Doctor Who story. He was notified by phone by Russell T. Davies that he had beat out 8,000 other competition applicants to appear on the program. Bell went on to a prolific acting career, which included the vaguely Who-like series Outlander (2014). The runners-up, Lizzie Watkins and Jonathan Wharton, appeared as extras.
This episode takes place just after the events of the Torchwood series-one finale End of Days (2007). At the end of that episode Captain Jack sees the doctor's severed hand glowing and hears both the sensors in it beeping as well as the sound of the TARDIS, and he runs out to find the Doctor.