Peter Capaldi’s final episode of Doctor Who this Christmas will feature Pearl Mackie as Bill Potts and be called Twice Upon a Time, it was revealed this evening (23 July) during a cast panel at San Diego Comic Con. Viewers will have to wait until Christmas to discover exactly how Bill, who will appear throughout the episode, makes her return.The magical final chapter of the Twelfth Doctor's (Peter Capaldi) journey sees the Time Lord team up with his former self, the first ever Doctor (David Bradley)'and a returning Bill Potts (Pearl Mackie), for one last adventure. The two Doctors find each other in an Arctic snowscape, refusing to face regeneration. Enchanted glass-like entities, who steal their victims from frozen time, and a World War I captain destined to die on the battlefield, but taken from the trenches to play his part in the Doctor's story. A tale about the power of hope in humanity's darkest hours, Twice Upon A Time marks the end of an era. But as the Doctor must face his past to decide his future, his journey is only just beginning.