It was 40 years ago that the world's most famous doctor made his first appearance. Childhood fan Nick Morrison looks at how a Time Lord captivated a generation.

DUMBA-DE-DUM, dumba-de-dum, dumba-de-dum, diddly-dum. It was tea-time on Saturday, November 23, 1963, and a legend was born. As the electronic music grinds out its remorseless rhythm - dumba-de-dum, dumba-de-dum, dumba-de-dum, diddly-dum, before a haunting wah-aaaah stretches through space - the swirling images on the screen part to reveal his name: Doctor Who.

When teachers Barbara Wright and Ian Chesterton follow their pupil Susan after school one evening, they are astonished to see her go into a police box in a junkyard. They are even more astonished when they step into the police box themselves, and find themselves confronted by a white-haired old man in a huge room of gleaming silver, flashing lights and humming consoles. It turns out it's not an ordinary police box at all, but a TARDIS, which, as everyone knows, stands for Time And Relative Dimensions In Space.

So began the Doctor's adventures through time and space that were to hold millions of children in thrall, as the TARDIS journeyed to distant planets and back and forwards in time on our own planet, battling foes, some in human form, others more ghastly still.

It was the Doctor's second adventure which introduced the world to what have become one of the most famous of all villains - the Daleks. Overcoming their inability to tackle anyone who could climb stairs, the Daleks have established themselves as one of the most fierce creatures in the universe, with their chilling cry of "Exterminate" heralding almost certain doom.

The Daleks have been followed by a succession of memorable monsters: the Cybermen, a race of part-men part-machine who have eradicated their emotions; the Ice Warriors, a militaristic people from Mars, and the Sontarans, a potato-headed warrior race.

And on top of these are those creatures which have made a one-off appearance, including disease-spreading maggots, strangling robots, Egyptian gods and giant, flying insects. A good rule of thumb was the uglier the monster, the more evil it was, a guide that also applied to the human adversaries. Shunting political correctness to one side, if a human was ugly he was bad; if he was deformed, he was bad and clever with it, and if he was in a wheelchair he was to be avoided at all costs.

But perhaps most evil, and certainly the most persistent, is the Doctor's arch-enemy, the Master. Like the Doctor, a renegade Time Lord exiled from his home planet of Gallifrey, the Master has dedicated his life to destroying the Doctor, and anything that gets in his way.

That first episode was broadcast the day after Kennedy was assassinated, and indeed was delayed for an extended news bulletin. But while an older generation can remember where they were when Kennedy was shot, I remember all too well where I was when the Sea Devils first walked out of the sea: behind the sofa. That image was imprinted on my four-year-old brain, and started an obsession which was to last into my teens.

And for the avid fan, there was no end of diversions. Of course, the programme was the thing - I remember crying when I thought I would miss an episode, but then I was only young and this was in an age before videos.

But there were also books, magazines, games, badges, dolls... yes, I had the Dr Who doll, as well as Leela, his bikini-clad assistant who was a big hit with the dads, a Cyberman and a talking Dalek, complete with a TARDIS with a revolving compartment. Put the Doctor in, spin the TARDIS, open the door and hey presto! he's gone. Spin again and there he is. I can't tell you how much fun that was.

A good guide to the age of any man of a certain vintage - and Dr Who is very much a male-thing - is to ask their favourite Doctor. Mine was Tom Baker, the fourth Doctor, the goofy one with a floppy hat, scarf and jelly babies. Those a little older swear by Jon Pertwee, the gadget-loving action man, or even Patrick Troughton, the recorder-playing clown, or William Hartnell, the grumpy old man.

A similar, and perhaps more heated, debate exists over the Doctor's companions, chiefly there either to provide muscle, or, more likely, a bit of glamour with the odd scream thrown in. Jo Grant has her followers, particularly after the actress playing posed nude after leaving the programme. The aforementioned Leela was very popular, and robot dog K9 was a novelty which swiftly became irritating. But surely head and shoulders above them all was Sarah Jane Smith, the journalist played by Elizabeth Sladen. Purely for the pitch of her scream, of course.

Many people see Baker's reign as the Doctor, the longest at seven years and 178 episodes, as the high point in the series, and Radio Times readers have just voted him the best-ever occupant of the TARDIS. Indeed many fans, myself included, feel the programme started to lose its way towards the end of his tenure. Whatever it was, it can't have been the fact we were growing up and noticing that the sets wobbled, the monsters were clearly men in ill-fitting rubber suits and the stories were desperate. And the fact remains that few people will choose Baker's successors, Peter Davison, Colin Baker, Sylvester McCoy, or the unmentionable Paul McGann, as their favourite Doctor.

Eventually, the powers at the BBC noticed this decline, a slide that couldn't even be saved by recruiting Bonnie Langford to stand by the Doctor's side, and pitting him against Nicholas Parsons. On December 6, 1989, after 26 years saving the universe, the last-ever episode was broadcast, although the production office didn't know it at the time. Plans for the next season were scrapped, and the TARDIS column rose for the last time.

Or at least it should have done. But some bright spark had the idea of making a movie-length version, starring Paul McGann, which aired in 1996. This was to be a very different Doctor, and not just because the film was rubbish and made the Children In Need spoof version look like a classic. When the Doctor kissed his companion, Grace, a dam was breached which will never be rebuilt. Millions of half-grown men will forever be wondering what he got up to with Sarah Jane once the TARDIS doors closed.

But there the story might have ended, on a woefully bum note, had not the BBC decided to regenerate the Doctor once more. Earlier this year, it was revealed that the ninth incarnation would appear on our screens in 2005, penned by Russell T Davies, the writer behind Queer as Folk and Bob and Rose.

Speculation is at fever pitch over who will take on the mantle, with names bandied about including Alan Davies, of Jonathan Creek fame, Withnail and I's Richard E Grant, and Bill Nighy, star of Love Actually and surely the best-suited for the role. There has even been talk of the next Doctor being a woman, but such sacrilege will surely come to nothing, if only because she will be less likely to have scantily-clad female assistants.

But then maybe some things are better left dead and buried. Maybe they don't need to be brought up to date, given a modern twist, made accessible for a new generation. Maybe some things are better left in the memories of boys who didn't notice that the caves were made of polystyrene, the alien landscape was a quarry in Wales and the monsters had zips on the backs of their necks. As the Doctor found out to his cost in Tomb of the Cybermen (1967), maybe it's best not to disturb that which you thought had gone for good.