Doctor Who Series 10: 12 Easter Eggs & References You Might've Missed In "Knock Knock'

12. "Grandfather"

When the Doctor helps Bill move into her spooky new student accommodation, she tells her friends that he is her granddad. When he says he'd prefer father, she covers herself by saying he likes to be called "Grandfather."

He certainly does, Bill. The Doctor's real granddaughter Susan always used to call him Grandfather when she appeared in the first year of the show before leaving the TARDIS in 1964's The Dalek Invasion of Earth.

This is the second time that Susan has been referenced in Series 10, after the Doctor had a framed portrait of her on his desk at St Luke's University in The Pilot.

The running callbacks to Susan have got some fans theorising that Bill might be a regenerated Susan. Well, weirder things have happened on this show!