The trailer for the forthcoming 'Doctor Who' Christmas special has arrived, and it's times like this that make us yearn for December. We already know that we'll get to meet Jodie Whittaker for the first time, but what final adventure is in store for Peter Capaldi's Doctor?

Mark Gatiss, Peter Capaldi and David Bradley star in 'Twice Upon A Time'Mark Gatiss, Peter Capaldi and David Bradley star in 'Twice Upon A Time'

Continuing on from season 10 episode 12, 'The Doctor Falls', we see the Doctor on an adventure with the first ever incarnation of himself in 'Twice Upon A Time'. Obviously, William Hartnell is no longer around to be in any more series, but David Bradley has turned out to be quite a convincing replacement.

Something has happened to time itself; in the Doctor's words, they're 'trapped inside a single moment'. Everything seems frozen somehow. Could it be something to do with the Doctor's consistent refusal to regenerate another time? One thing's for sure, there's a World War One soldier (credited as simply The Captain) who desperately needs their help, and he's brilliantly played by occasional writer Mark Gatiss in his third onscreen 'Doctor Who' appearance. Plus, Bill Potts (Pearl Mackie) is back for one more trip with her lecturer. 

The new episode is being directed and written by Rachel Talalay and Steven Moffat respectively, both of whom were at the helm of the previous two episodes. The trailer for 'Twice Upon A Time' was unveiled at San Diego Comic-Con when the cast and crew appeared on a panel to talk about the new installment on Sunday (July 23rd 2017). It has been interspersed with footage from the original series of 'Doctor Who', including a glimpse of the Doctor's companion from the original season 4, Polly (Anneke Wills).

More: Will Kris Marshall play Jodie Whittaker's assistant in 'Doctor Who'?

'[It's] a Christmas episode without being overtly Christmassy - it's very happy-sad', Mark Gatiss explained. '[It's] a fantastic episode and we had a great time doing it. It was a lovely way out.'