Doctor Who: on the set of the 50th anniversary episode He has written the anniversary episode himself, and promises, 'It's not just about honouring the past, though we do do that. I was aiming for... how about the most important day of the Doctor's life? The adventure that actually has an impact on him. I think most of his adventures make an impression on everybody else - but he probably forgets them 10 minutes later. Matt Smith says that fans won't be disappointed. 'What's really interesting is that Steven has managed to invent something new about the character, which is wonderful after 50 years," he says. 'And it's a testament to the format, and to the idea, and to the nature of the character and the show. The result is a feature-length episode called The Day of the Doctor, which the BBC is turning into a global event by ramping up anticipation among fans around the world. It is to be broadcast simultaneously in 200 countries. A teaser trailer has been available for three weeks, but today will see the first broadcast of any trailer that includes footage from the episode itself. Perhaps the biggest clue as to the direction the story would take, though, came at the end of the final episode of the last series. In a convoluted and fantastical story the Doctor and Clara had time-travelled to the Doctor's own grave and, once there, found themselves caught in a sort of vortex that represented the 'time stream" of the Doctor's life. David Tennant, Hurran and the director of photography Neville Kidd. Photo: Jonathon Williams There, the shadowy figures of previous Doctors - such as Hartnell and Peter Davison - flashed past. At the end - and baldly introduced by an on-screen credit as 'the Doctor" - they found John Hurt. 'He's from the Doctor's past, and the Doctor did not expect to see him again," Moffat says. More will be revealed about Hurt's character in this new anniversary episode. We pick up, as it were, months later. So you'll get an update on what Clara's been up to, a little bit about what the Doctor's been up to. And then, as will tend to happen with multi-Doctor stories, things go a bit haywire. It will feel, at the beginning, like we get three different adventures with three different Doctors," Moffat continues. And these connect, to become the same adventure. Very, very roughly, I suppose, and with apologies to Charles Dickens, it's the A Christmas Carol structure - there's the ghost of the past, the present and the future. There's plenty of inter-Doctor rivalry and, as the Doctor's 50th-birthday present, we're establishing a whole new Doctor right there in front of you. The pivotal scene about to be filmed calls for all three Doctors - Smith, Tennant and Hurt - as well as their companions, Coleman and Piper. As the technicians prepare for the cameras to roll, the actors chat away easily. (The potential for rivalry between actors as well as characters was, Tennant says, something he feared before coming back. It could have been difficult," he says. That was my one nervousness - not about Matt per se, but just that he would have every right to feel like his territory was being in some way infringed. But he doesn't. We're having a right laugh, actually.") Tennant, Piper, Smith and Jenna-Louise Coleman. They are all standing in a sort of interplanetary shed. It has a vaulted wooden frame, walls of wattle and plank, and a floor of dirt and twigs punctuated by golden glitter. This, apparently, is the scene for a great Doctor Who moment. 'It's in the middle of a strange desert - out of time, out of place," says the show's production designer Michael Pickwoad (whose father, it transpires, was an actor called William Mervyn, who appeared as a guest star in the 1960s). 'So it's like a barn. But it's got a slightly sort of Hobbit-like quality - is it in the imagination, is it real, is it not? It's not quite like any barn I've ever been into. Smith's and Tennant's Doctors have each arrived in the space-barn in their own Tardis, with Tennant's a slightly muckier shade of blue. At the other end of the barn stands another device: a red jewel, like a huge ruby, sits at chest height on top of a metallic plinth, which is decorated in burnished silver and gold. As the actors block the scene, their hands keep reaching as if to press down on the jewel. As filming finally gets under way, the dialogue is loaded with moral ambiguity. Since that first, fleeting appearance, Hurt's character has been described as a 'dark Doctor," a hitherto suppressed incarnation of the show's central character. 'You were the Doctor on the day it wasn't possible to get it right," Smith's Doctor says, trying to comfort Hurt's, whose face is etched with inner pain. As the three Doctors gather around the jewel, it also seems that pressing it down would have terrible consequences. 'What we do today, we do not out of fear or hatred, but because there is no other way," Tennant's Doctor says portentously. Inside a Tardis. Since Doctor Who returned with Russell T Davies in charge in 2005 (having been cancelled because of poor quality and ratings in 1989) every episode has contained plenty of CGI. This one is also being filmed in 3D and will be screened, within a day or so of the TV broadcast, in cinemas worldwide (no fewer than 87 have signed up in Australia alone). On set, the director Nick Hurran and his team - especially the stereo supervisor Adam Sculthorp, so titled because two cameras must shoot side by side to create the 3D effect - agonise over the look of the scene. 'We're trying to make it a natural experience," Sculthorp says. He argues that 3D done well enhances a film's story. The red jewel, for example, will be pulled forwards into what Sculthorp calls 'theatre space" - a term coined to describe an extreme 3D effect that makes an object appear to leap out of the screen. 'We've used the "stereo depth" in that shot to hopefully take the viewer and make them feel how important it is," he explains. For Matt Smith, the extra effort involved in 3D filming is important - but 'laborious. Smith and the other actors also face another problem on this particular set: the heat (especially Hurt, who looks fit to expire). Illuminating the space-barn mainly from the outside requires an enormous lighting rig - one of the biggest ever used on the series - which hangs above it in the cavernous space of the studio ceiling. John Hurt filming a pivotal scene. Tantalisingly, the studio has massive vent tubes snaking down from its hangar-like roof. But, today at least, they fail to provide any relief from the heat. 'Don't get me started on the lack of air conditioning in the studios," Smith says good-naturedly. The vents might as well have a fairy blowing warm air out of them. It's ridiculous. But there we go. You've got to march on, you've got to get on with it. After the anniversary show Smith has only one more episode to go in his Doctor's trademark bow tie - during the Christmas special, he will give way to the 12th (or perhaps 13th, given Hurt's intervention) Doctor, Peter Capaldi. Capaldi's arrival has been widely welcomed, but fans have anointed Smith a highly successful Doctor, and he will be missed - not least by his own mother. My mum was desperate for me to stay, and really, really, really tried to persuade me," he says. But he thinks that the show deserves 'a new lick of paint" and that it's the 'right time" for him to leave. 'Some people really love you, some people really don't love you," he says. And so on Christmas Day Smith will dissolve into Capaldi, just as on New Year's Day 2010 Tennant dissolved into Smith. Steven Moffat, the executive producer and lead writer, wearing 3D glasses. Four years on, Tennant remembers the experience of making that handover episode. It was a very odd sensation," he says. Because it feels like an event, filming a scene like that. But then, of course, you leave. And even as you're walking out of the door, you can hear the crew going, "Right! So, rehearsing..." And you think, wow! It's the definition of "dispensable," isn't it?" He pauses, and then a broad grin spreads over his face. Which, of course, is why Doctor Who has managed to struggle on for 50 years. Doctor Who: The Day of the Doctor is on BBC One on November 23