Doctor Who Series 10: 7 Big Questions After 'Knock Knock'

The idea that Bill could be a fob-watched Susan will have been given an added boast by her decision to address the Doctor as grandfather - the slightly old-fashioned term that calls to mind Carole Ann Ford’s Susan. She was still using the same title for the Doctor on her last comeback to the show in The Five Doctors (1983).

Could the vault be a red herring, with Bill’s identity the real secret that the Doctor is trying to protect? With more reminders of William Hartnell’s original Doctor in Knock Knock, including another line of dialogue about him stealing the TARDIS from Gallifrey Susan Foreman is almost certainly involved in the Twelfth Doctor’s final days in one way or another.

Bill’s story might of course turn out to be an all too familiar and human one. The reminder of Bill’s biological Mother, when a proud Bill talks to her treasured photograph, suggests that this part of their family history still has some mileage. The whole raison d’etre of David Suchet’s character is to make sacrifices to keep his Mother alive and there is a very real possibility that this is foreshadowing things to come. Could we see a surprise appearance from Bill’s mum, whose image is beginning to stick with us, and might Bill’s fate be caught up with trying to save her?