In a dramatic episode full of fire, tears and the four knocks of doom, David Tennant’s Doctor finally regenerated, bringing to an end what is arguably the most successful era of the show ever.

For months now, in a series of special episodes, the show’s executive producer and lead writer Russell T Davies has been upping the tension towards yesterday’s climax, and although some viewers have apparently tired of Tennant’s omni-

presence in the festive schedules, last night’s farewell was typical -- brilliantly typical -- of what Davies does best: it was funny and sad, scary and uplifting, threatening and comforting.

It was drama on the verge, and utterly -- thank goodness -- unlike anything else on television.

There was a threat to the universe, of course (even if the planet-in-the-sky device seemed a little familiar) but something more important was happening: this was the Doctor saying goodbye to his friends, and to himself.

At times, the goodbye felt a little long, but the out-of-nowhere cameos from the likes of Elisabeth Sladen and Billie Piper were unexpected and delightful, although, considering she’s been one of the best companions ever, it was a shame Catherine Tate was having a kip through the whole thing.

As for Bernard Cribbins, where has he been all these years? He’s been utterly compelling as the Doctor’s companion Wilf (and isn’t it brilliant when everyone on television seems so young to have a hero who is so old?)

Perhaps last night’s episode would have benefited in places from being squeezed and tightened, but as always Davies got the right balance: when it teetered on the edge of taking itself too seriously, Davies adjusted the controls and threw in a great joke (“Cacti? That’s racist!”). And showing just how much he understands this wonderful, batty, British science-fiction show, he threw in the best scene of all when the Doctor’s finger was hovering over the trigger of a gun and he didn’t pull it, because he couldn’t.

Last night was the end of a long wait for fans. It was in October 2008 that Tennant announced he was standing down from the role he’s played for five years. He has unquestionably been a success for the show, arguably more so even than Christopher Eccleston, who revived the programme in the first place, but he’s probably leaving the part at just the right time.

He is wildly popular among younger fans -- he was recently voted the best Doctor ever by readers of Doctor Who Magazine, managing to unseat the great Tom Baker-- and viewing figures have been going up and up. And besides, the Doctor should always bow out before he becomes predictable.

We only got a brief glimpse last night of Matt Smith, Tennant’s successor, but it was an encouraging one. In the few seconds we saw him engulfed in flames and chaos and trying to prevent the Tardis crashing to Earth, he was dotty and bonkers and hyperactive and full of fun: in other words, exactly what the Doctor should be.

Smith will take over full-time in the spring (as will the Scottish writer Steven Moffat who takes over from Davies as lead writer and executive producer) and whatever happens, his age alone means he will be a very different kind of Time Lord. Smith is 27, making him the youngest actor ever to play the role. He will have a young companion too, Amy Pond, who, in another touch of Scottishness for the show, will be played by Inverness-born Karen Gillan.

David Tennant has been mostly brilliant (a bit like Doctor Who itself), and last night’s farewell was epic, funny, shocking and tearful. But remember: the young Scottish actor turned into a star by the old story of a Time Lord is already off having another life, filming the pilot of a new drama in America, so he’ll be just fine.

And remember this as well: this is Doctor Who, the show that specialises in rebirth, and those of us who love it are never sad for very long.