The story was filmed as part of the first block of stories but a decision was made to hold it over as the opener for the second series.
An emergency klaxon is heard in the TARDIS. The Cloister Bell is first heard in Logopolis: Part One (1981).
This episode takes place in England in 1964.
This is the first time the Doctor manages to return to contemporary Earth since An Unearthly Child (1963), the first story.