Steven Moffat and Peter Capaldi's final episodes of Doctor Who feel like a time for revisiting old friends – not to mention old enemies – and for going back to the beginning.
We already know that the departing showrunner is giving his last Doctor the parting gift of a return for the Mondasian Cybermen – Capaldi's favourite monsters and one of the show's seminal villains – as well as another classic foe in the form of the Martian Ice Warriors, and that he'll also be bringing back John Simm's maniacal incarnation of the Master, last seen seven years ago.
So you'd be forgiven for wondering what other blasts from the past Moffat has up his sleeve. And if you're the kind of person who's on the lookout for these sorts of things – a hopeful romantic or a conspiracy theorist, depending on how you look at it – you'll probably have noticed some references to another original Doctor Who character in the new series.
It's been impossible to miss the framed picture of the Doctor's granddaughter Susan on the desk of his study at St Luke's University, where he's currently a resident lecturer. She is of course the eponymous Unearthly Child of the first ever Doctor Who story, who travelled with her grandfather, the First Doctor.
Moffat has said that the Doctor has her photo on his desk as part of his attempts to blend in while he's (relatively) grounded on Earth, guarding whatever it is that's inside that mysterious vault beneath the university.
But he also admits that the new series is as close to a reboot as new Who has come, going back to the beginning in a way that means "you can jump on with that [opening] episode never having seen it". And the beginning was of course the Unearthly Child, Susan.
Episode four, Knock Knock, chucks in another reference to her, drawing a parallel between Susan's relationship with the First Doctor and Bill's with the Twelfth.
When an embarrassed Bill tries to explain away the presence of the Doctor to her new housemates as he's helping her to move in, she tells them "he's my granddad", to which the Doctor replies "Oh, grandfather at least, please". And from then on we get to hear something we haven't heard on Doctor Who since Susan's brief reappearance in The Five Doctors in 1983 – the Doctor's companion calling him grandfather. Hard not to feel a little pang of nostalgia from that.
So what does it all mean? Is Moffat preparing us for the return of Susan? Or is he simply enjoying going back to basics, echoing the first ever relationship in Doctor Who as he prepares to say goodbye?
You could argue that there have been similar call-backs to another character – Moffat’s own creation, River Song. The Doctor's wife is the other family member who takes pride of place on his desk and he must have been thinking of her when he took Bill to the same Frost Fair where he and River had once been ice skating for her birthday. But then some people think we haven't seen the last of River either.
There's one more compelling reason to wonder whether Susan might be returning, though, and that's the rumour about the Christmas special. I wrote here about how David Bradley could be reprising his role as the First Doctor (from Doctor Who ‘making of’ drama An Adventure in Space and Time) to join forces with the Twelfth for his last adventure. The BBC won't confirm or deny it (which some would say speaks volumes) and although the possible explanation I put forward for how it might happen doesn't include Susan, that was educated speculation at best and it's not mad to think that where the First Doctor might go, his most faithful companion might follow.
That would be interesting for lots of reasons, not least to see the Twelfth Doctor's reaction when he is reunited with his granddaughter after 1,000 years.
Capaldi himself is keen, answering unequivocally when asked by a fan last year which companion he’d like to bring back: “Susan. Her story wasn’t finished”. And speaking to Doctor Who Magazine, Carole Ann Ford, the original Susan, remembers a recent Doctor Who set visit when Capaldi “was running around the Tardis saying, ‘She’s got to come back! She’s got to come back!’”
Would she share his enthusiasm? “Yeah, of course! Are you kidding?”
Ford’s agent backs that up but tells me there’s been no contact yet, and given that David Bradley is the actor mooted as the First Doctor, and that he may well be operating in the Modasian era, perhaps it’s more likely that Claudia Grant, who played the young Susan alongside him in An Adventure in Space and Time, will be the one returning.
Would she be available?
Interestingly, her contract for Harry Potter play The Cursed Child has just come to an end – she is one of only a few of the supporting cast not to be continuing – and I’m told she has a space in her diary beyond that.
Of course, Susan is a Time Lord/Lady, so depending on the specifics of the story she may have regenerated since we last saw her and could have any face at all. But if she is set to appear alongside the First Doctor – and remember, this is just speculation on my part – you’d have to assume it will be the ‘original’ Susan. In which case, my money would be on Claudia Grant appearing in Steven Moffat’s last episode as the Doctor’s granddaughter, giving Peter Capaldi a farewell gift and Christmas present rolled in to one.
Doctor Who series 10 continues on BBC1 on Saturdays