David Troughton (Professor Hobbes) is the son of Patrick Troughton, who played the Second Doctor in Doctor Who (1963) from 1966 to 1969.
This is the first complete story where the TARDIS does not appear since Genesis of the Daleks: Part One (1975) (in the original series, there were usually far more episodes in a story and it was common to have episodes without the TARDIS).
This was set to be episode eight of season four with Silence in the Library (2008) and Forest of the Dead (2008) following it. Had the episode been originally where it had been placed it would have been the 50th episode of the revived series. A reference to this milestone was seen in the bus' name "Crusader 50".
Disappearances references: Dee Dee tells The Doctor that she has written a paper on the lost moon of Poosh. Whilst speaking to Sky, The Doctor mentions the Medusa Cascade.
The first time on the show someone mentions "He will knock 4 times."