If the rumours are true, this year’s Doctor Who Christmas special could be a rare double Doctor episode, with Peter Capaldi’s departing thirteenth incarnation being joined by the First Doctor, played by An Adventure in Space and Time star David Bradley.

It’s a mouth-watering prospect for classic Who fans, and would surely be equally so for Capaldi himself, whose devotion to Doctor Who began as a youngster watching original First Doctor William Hartnell.

BBC sources remain non-committal about the suggestion but for me it has the ring of truth, for several reasons…

Showrunner Steven Moffat has previous on this, having masterminded the first multi-Doctor episode for almost 30 years when he brought together Matt Smith’s Eleventh, David Tennant’s Tenth and John Hurt’s War Doctors for 50th anniversary special The Day of the Doctor (that’s unless you count 2007 Children in Need skit Time Crash which, yes, was also penned by Moffat).

And Moffat is placing Capaldi squarely in First Doctor territory, just in time for the Christmas special, by bringing back the original Mondasian Cybermen for the series 10 finale.

The Mondasians are a favourite of Capaldi’s and Moffat has admitted that their return is at least partly a farewell gift to him. So it doesn’t seem too far-fetched that he might also give him the Christmas present of appearing alongside one of his best-loved Doctors.

True, festive episodes of Doctor Who are often fluffy, family-friendly affairs, rather than geek-fests, but I get the feeling that a demob-happy Moffat – who is leaving at the same time as Capaldi – is ready to sign off with something that will please himself and other hardcore Who fans.

The storyline we’re hearing about would certainly fit that bill. It supposedly sees the First and Twelfth Doctors teaming up to save Gallifrey by moving it to another dimension, something that we know takes place – with help from the other 11 Doctors – in The Day of the Doctor. But it seems that in this case we’d be seeing it from the points of view of One and Twelve, rather than War, Ten and Eleven.

And just to add to the fun and games, we’re told that the first glimpse of Capaldi’s attack eyebrows in The Day of the Doctor was actually the prelude to his regeneration, which will then play out in the Christmas special.

It’s a neat tying up of loose ends that sounds too good not to be true.

And yet one question remains: exactly where in the First Doctor’s timeline does all this take place?

Looking back at canonical adventures – and adding the Mondasians to the equation – there’s one glaring hole that fits the bill…

The Mondasian Cybermen appeared in just a single Doctor Who TV story, The Tenth Planet. It was the last appearance for William Hartnell, who was by then suffering from ill health and hardly had the energy to deliver his lines, never mind run down corridors being chased by aliens.

Hence, it became one of the first Doctor-light stories, with companions Ben and Polly doing most of the legwork and the Time Lord’s absence explained by the rather dubious idea that he was either lounging around or imprisoned by the Cybermen.

But what if that was just a red herring? What if, during that time, the Doctor had actually hopped into the Tardis and headed off for one final adventure with a later version of himself?

Moffat has already shown he has a knack for spotting a blank space in Doctor Who history and creating something to fill it. Arguably the biggest and best idea since the show returned in 2005 is the revelation that there was an unseen regeneration between Paul McGann’s appearance as the Eighth Doctor in the TV movie and Christopher Eccleston’s debut as the Ninth Doctor.

So the idea that the Christmas special will take place during that mysterious period when the Doctor was absent for the majority of his last adventure is both very appealing and very believably Moffat.

I hope it’s true. It would make a fitting swansong for a pair of Doctor Who fanboys – the departing showrunner and the Twelfth Doctor – even if it is a shame that Capaldi will never get to act alongside William Hartnell himself…

Series 10 of Doctor Who continues on Saturday nights on BBC1