Just what the Doctor ordered: How David Tennant's Time Lord saved Auntie

David Tennant's reign as the Doctor saved the BBC – and it comes to a dramatic end on New Year's Day. But fear not! The next Time Lord, Matt Smith, will have plenty on his plate: the Second World War, Van Gogh...

Do you hate David Tennant? Then this will be the worst Christmas of your life. You might as well gaffer-tape your face until January, because between today and New Year's Day, that lanky Scotsman with the Converse tennis shoes and the pinstripes and the great hair-wax explosion will fill more airwaves than Fiona Bruce and the jewellery demonstrators of QVC combined.

Doctor Who will drive the Tardis-like Santa's sleigh all over the BBC One channel idents. Off he'll go, with the full complement of reindeer, to score a big "O" into the winter sky – and he'll make around 40 more house calls during the festive season. Principal among these, of course, is his 10th incarnation's final two-part adventure. (Its title, "The End of Time", is sufficiently apocalyptic to suggest that Tennant's Doctor will not, like Colin Baker's, perish simply by falling over and banging his head on a hard bit of the Tardis.) Each episode will be followed by Doctor Who Confidential, a supporting documentary on BBC Three, which will comprise behind-the-scenes footage and the now-customary interviews with production staff overusing the word "iconic". A CGI Tennant will battle monsters in the New Mexico desert in the animated story "Dreamland", and the flesh-and-blood original will be standing in for Jonathan Ross on Radio 2, lolling on the sofas of GMTV, Blue Peter and Alan Carr; hosting a Who-themed Never Mind the Buzzcocks, warming a seat on QI and Desert Island Discs, suffering the slings and arrows of outrageous fortune as Hamlet on BBC2, and reading ' a week of bedtime stories on CBeebies. (That may be it, but the details of the Queen's Speech have yet to be announced.)

Fortunately for Tennant, the British nation has fallen hopelessly, madly and devotedly in love with him – and the 900-year-old Time Lord whose hair products he's been using for the past five years. He is everywhere. Doctor Who is everywhere. Its language, its concepts and its characters have become part of our national conversation. Its season finales harvest ratings of the sort once thought lost to television drama. Posters outside churches read "Christ – the original Time Lord", suggesting that the Church of England believes that Doctor Who is bigger than Jesus. Those of us who, 20 years ago this month, were among the small band of die-hards who watched Sylvester McCoy stroll off into the sunset with a swing of his question-mark umbrella are still pinching ourselves. Two decades ago, the BBC regarded Doctor Who as an embarrassment: something to schedule against Coronation Street in the hope that the ITV gargantuan would roll over and squash it. Now the Corporation uses the programme to justify its existence to the public and the Government. How did this happen, exactly?

The question still occurs to some of those who brought about the show's revival. When the writer-producer Russell T Davies persuaded the BBC to let him make 13 new episodes of Doctor Who with Christopher Eccleston in the lead, he constructed them as a stand-alone series, and was fully prepared for it to be his sole chance to see his name and the Doctor's on the same credit-roll.

Doctor Who did not return easily to the world: during the first few months of filming, its personnel were forced to learn from scratch the forgotten art of making a drama series that demands a weekly parade of monsters, an inferno of pyrotechnics, and the realisation of images that are much easier for writers to type than technicians to materialise: a swarm of Daleks swooping over the burning carcass of the planet Earth; the heat-death of the solar system; a Second World War gas mask erupting from the mouth of an old man; Number 10 Downing Street obliterated in a missile strike. It was not a guaranteed success. Five years later, however, Christmas seems unimaginable without an hour spent watching Tennant tackling something evil against a flurry of artificial snow.

There's a myth that's grown up around the new Who: that it has achieved its success in spite of the old series from which it sprang. And at some level, the brand managers at the BBC concur with this uncharitable analysis: old Doctor Who is never repeated; images from the programme's deeper past rarely feature on Doctor Who Confidential; the range of novels featuring the adventures of the old Doctors was quietly killed. Which is a shame, because that view occludes the fact that much of what seems most innovative in new Doctor Who is deeply rooted in its history – and not always the best-regarded periods of that history. Those elements of the revived series that seemed most fresh in 2005 – a working-class companion with a well- developed family background, a council-estate setting, the sense that the Doctor's personality contained a thick streak of darkness – are all to be found in Sylvester McCoy's last season of Doctor Who. McCoy's final story, a parable about Thatcherism set in a frowsty tract of west London, written by Ken Loach's screenwriter Rona Munro, wouldn't require much revision to serve as a script for Tennant.

And the endgame for the 10th Doctor – which began in last month's special "The Waters of Mars" and will conclude the incumbency of both Tennant and Davies – follows an even older set of moves, ones laid down in the prehistory of the programme. "The Waters of Mars" began with Lindsay Duncan's space commander battling an alien influence that infected her colony's drinking water and caused her colleagues to transmogrify into goggle-eyed zombies whose cracked faces issued alarming torrents of clear liquid. By the end of the story, however, her enemy had become the Doctor himself. It was Tennant's finest hour: before our eyes, his trademark cheekiness gained an unattractive note of contempt; his bravado soured into a form of self-love. And all because he had taken the decision to defy the Laws of Time and save some hapless humans from the fate history had reserved for them. Because he had dared to play God.

Similarly, in a story from 1978, Tom Baker's Doctor found himself in possession of an artefact called the Key to Time, which would allow him to manage the moral life of the universe. He rolled his eyes and declared his desire for divine power. But he was bluffing. Tennant's Doctor, full of hubristic pride in his ability to outwit the processes of history, meant it.

It felt like a big switcheroo, but really it was the moment at which Davies made explicit an idea that has haunted Doctor Who since William Hartnell stepped out of a London fog 46 years ago – that without the influence of his companions, the Doctor is not a nice person to be around; that he needs a human sidekick to curb his excesses.

In the very first story, back in 1963, there's a moment at which one of the Doctor's companions – Ian Chesterton, a heroic school teacher played by William Russell – catches the Doctor contemplating a shocking act of violence. It's 100,000 BC and the Tardis crew has just watched a caveman being mauled by a sabre-toothed tiger. The Doctor wants to head back to the safety of his police box and leave this Neolithic casualty to die. But his three fellow travellers agree to tend the wounded man and risk the possibility of being recaptured by the less conciliatory members of his tribe. Moments later, Ian discovers the Doctor kneeling over the caveman with a pointy rock in his hand – and is clearly not convinced by his blustering explanation that he intended to persuade the tribesman to draw a map in the sand to guide them home.

In the following story, the Doctor sabotages the Tardis because his companions don't share his enthusiasm for exploring a city in the middle of a petrified alien jungle. Soon, all four are expiring from radiation sickness and languishing in a metal cell provided by the hidden inhabitants of that city: shouty types with sink-plungers where their left arms should be. The lesson of the first series of Doctor Who is clear: the Doctor needs humans to keep his conscience in good order.

Unlike any previous actor to play the role, Tennant is a Doctor Who obsessive, with a spooky command of the arcana of the television series in which he currently stars. I'm sure that when he acted the shocking finale of the 2006 Christmas special "The Runaway Bride", in which the Doctor, a crazed and vengeful glint in his eye, commits infanticide upon the brood of a spidery alien queen, he was aware that the Doctor's moral history contains many such moments. The Doctor is a man who has preached the sanctity of all life, committed genocide and blown up inhabited planets. He has torn a strip off companions who have attempted to change history – and done his own share of meddling. And soon his Doctor will be paying with his life for that interference. He has over-reached himself, and the universe will punish him for it by killing him. This Doctor – much as we loved him – had it coming.

He will, of course, survive this death. By New Year's Day we will have glimpsed his replacement. Matt Smith is currently filming a new series of Doctor Who under the eye of its new head writer Steven Moffat, who knows the programme as well as his predecessors and has already written some of its best episodes. The new team, of course, don't want to go down in history as the people who screwed up the BBC's biggest success this decade.

But it's hard to imagine it going too badly wrong: the new season will feature the return of the Angels, those sinister stone creatures from the story "Blink". It will send the Doctor back to the Second World War to tussle with the biggest politico-moral dilemma the show has yet treated. It will bring the Doctor face-to-face with Vincent Van Gogh in a story that its author, Richard Curtis, claims will terrify us all. Most of all, though, Smith seems a perfect piece of casting: he looks like a boy who might grow up into William Hartnell.

I have a prediction: it will be the best season of Doctor Who ever broadcast. But even if the public don't agree; even if the new production team discover that when your predecessors achieve audiences of 12 million the only place to go is down, they will not destroy Doctor Who. That's the nature of the achievement wrought by Davies and Tennant.

Today, Doctor Who is more than a TV series. It's something we do. It's a story that will go on being told even when television falls out of love with it, and the chat-show hosts and the quiz-show bookers no longer call the production office. No one can kill Doctor Who. Not now. Not the BBC. And not the gods.

'Doctor Who: The End of Time', 6pm on Christmas Day and 6.40pm New Year's Day

Doctor Hubris: Why Who had it coming

'The Christmas Invasion'

David Tennant's first story, and he's already judge, jury and executioner. When the leader of the skeletal Sycorax attacks him from behind, he sends him plunging to his death with a well-aimed satsuma. "No second chances," he spits

'Tooth and Claw'

The Doctor saves Queen Victoria from an alien werewolf, but does it in a fashion too smug for her taste. She banishes him for his levity in the face of death

'The Runaway Bride'

The Doctor is visibly vengeful as he sends the babies of a giant alien spider down a giant plughole in the River Thames

'Forest of the Dead'

In a vast library, the Doctor taunts a race of flesh-eating creatures: "Look me up!" It's the kind of pride that comes before a fall

'The Waters of Mars'

The Doctor decides he no longer needs to obey the Laws of Time. A meeting with his Nemesis is on the cards... MS