This year's Doctor Who Christmas special, "Twice Upon a Time," has been revealed. Peter Capaldi, the 12th Doctor, will join David Bradley, portraying the first Doctor, for a moment stuck in what appears to be WWI.

The trailer, shown directly after the Doctor Who panel at San Diego Comic-Con, shows both the first and 12th doctors uniting, trapped in a single moment in time. The trailer shows two men on a battlefield, frozen, pointing guns at each other. The camera cuts to a solider, played by Mark Gatiss, who asks Capaldi and Bradley if they happen to be doctors. The following camera cuts show both doctors working together, because of course, two is better than one.

Pearl Mackie, who plays Bill Potts, will return for this episode but will not be returning for the next season as a companion. "Twice Upon a Time" will be Capaldi's final episode as well, ending his three-season long run as The Doctor.

As for Gatiss, this episode marks his fifth appearance in the show. In the past, he's played minor roles such as Lazarus in "The Lazarus Experiment," Danny Boy in "Victory of the Daleks" and "A Good Man Goes to War," and Gantok in "The Wedding of River Song." This time around Gatiss will be playing 'The Captain,' though we're not quite sure what that means just yet.

The forthcoming Doctor, Jodie Whittaker, is believed to make her debut as the show's first female regeneration in this year's Christmas special, but she was not shown in the teaser trailer. "Twice Upon a Time" will air this Christmas Day on BBC.