DAVID TENNANT made his debut as the tenth incarnation of the Doctor back in 2005, as a trench- coated, trainer-wearing version of the 903-year-old Time Lord.

Taking over from Christopher Eccleston, Tennant’s energetic, lightly comic portrayal quickly secured a legion of loyal fans. Understandably, the news in October 2008 that Tennant was leaving the show created palpable shockwaves among the series’ followers.

Now the time to say adieu to Tennant’s Doctor is nearly here.

The second of four specials that mark his departure airs on November 15, with the final two parts to be shown over Christmas and the New Year period, when Tennant will regenerate into the eleventh Doctor, to be played by Matt Smith.

As befitting the end of an era, the second of the specials, entitled The Waters Of Mars, which is co-written by Phil Ford and Russell T Davies, who is also leaving the show, is set to be one of the most terrifying and dramatic yet.

The teaser trailer reads: "Mars, 2059. Bowie Base One. Last recorded message: ’Don’t drink the water. Don’t even touch it. Not one drop’." It’s an episode which sees The Doctor and his companion Adelaide, played by the acclaimed actress Lindsay Duncan, face true terror on the Red Planet.

"It tells a different sort of story I think," says Tennant, all wide eyes and jerky gestures that lend him a slightly hyperactive air. "It’s a long time before the Doctor takes control and I think that’s what’s unnerving about it, it’s not the standard kind of structure," he adds.

"We expect the Doctor to take control earlier, we expect the Doctor to know how he’s going to do it and how he’s going to win through and he sort of doesn’t. And even though he wins in the end, it’s a bittersweet victory. And people die and that’s part of the story we had to tell, to go where we’re going. It’s intense."

Tennant admits there’s a sense that the tenth Doctor’s days are numbered. "Storm clouds hang over the last series," he’s said. "Things start to happen that mean things can never be quite be the same again."

As tough as it was for the 38-year- old Scot to decide to leave when he did, he says he’s pleased his time as the Doctor is satisfyingly wrapped up, while hinting at a new future.

"Yeah, it feels like we need to have a big ’end of an era’ story and to get on, which I think is important in this show which, let’s face it, has been going a long time before we showed up and no doubt will carry on into the far distant future. I think we tell a big old farewell story and then hand it over in rude health."