LONDON - For millions of Britons, a traditional Christmas Day includes turkey, brussel sprouts, Christmas pudding - and a special seasonal episode of Doctor Who. The global success of the venerable sci-fi series means that fans around the world will also tune in Monday to watch Peter Capaldi's final adventure as the space-hopping Time Lord known as the Doctor. The show has had a dozen Doctors over its 54-year history, and this is one of those bittersweet moments when one lead actor hands over to another. At episode's end, viewers will see Capaldi transform - through a Time Lord process known as regeneration - into Jodie Whittaker, the first woman to play the part. Before that comes a rollicking and poignant adventure that moves from the World War I trenches to the South Pole, features mysterious creatures made of glass and (of course) involves a jaunt in the Tardis, a time-and-space machine shaped like an old-fashioned British police phone box. The episode loops back into the show's own history, featuring an appearance by the very first Doctor, a white-haired gent who appeared on TV screens in 1963. He was played by William Hartnell, who died in 1975; David Bradley fills the role here. Mark Gatiss, a regular writer for the show who guest stars in the episode as a British Army officer marooned in time, said there was a valedictory feel on the set of the finale. The sense of eras ending all over the place was very profound but at the same time it was actually a very happy experience, Gatiss said in an interview. - AP