There’s a double goodbye in this year’s annual holiday Doctor Who special, which makes it even more noteworthy than usual. It’s the final appearance by Peter Capaldi, who’s the twelfth actor to play the title role in this long-running series – and it’s also the final script by Steven Moffat, who’s been running the show brilliantly for years now. But there are firsts, too: It’s the first appearance by the 13th Doctor, played by Jodie Whittaker – the first woman to play the role of the time-traveling, shape-shifting Time Lord.