The latest episode of Doctor Who, Knock Knock, is also the latest episode of Steven Moffat's era that aims to make us terrified of something fairly innocuous and everyday. This one focussed on creaky floorboards but previous examples include stone statues in Blink, cracks in the wall in The Eleventh Hour, Snowmen in, er, The Snowmen... Even WiFi in The Bells of Saint John. Or puddles in The Pilot.
That running theme is not the only callback to be found in the episode, however. Just like the previous instalments of Series 10, Knock Knock was creaking - see what I did there? - under the weight of so many allusions to both Doctor Who episodes past and present and other classic TV shows and movies.
As it was a spooky episode, there were a few nods to famous horror movies in there. For instance, a character supernaturally sinks into the stairs at one point, echoing a famous moment from the original A Nightmare on Elm Street, but there are plenty more where that came from.
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!