"My name is William Hartnell and as Doctor Who I make my debut on Saturday 23 November at 5.15."

The world's unlikeliest and best S.F show is 45 years old. In 1963, incoming Head of Drama Sydney Newman decided that the BBC needed a new S.F. show to fill the early Saturday evening slot. Designed by committee, the show was eventually commissioned as a 13-episode serial featuring a bad-tempered professor-type from another world in charge of a capsule that could travel through space and time. Successive serials would see "Doctor Who" and his companions travel to other worlds, to significant points in Earth's history, and "sideways".

The show's mythology developed over the years as successive producers brought their own ideas, but the biggest and most important ideas were, firstly, the TARDIS, a time-travel capsule forever disguised as a 1930s-style London police box, and, in 1966, the decision to incorporate the recasting of the lead role into the plot of the show. Whereas Hartnell's grumpy old curmudgeon had evolved into a much-loved, stern but kind-hearted grandfather figure, Troughton played a radically different character. Troughton's Doctor, initially wearing a Chaplinesque variation of Hartnell's Edwardian get-up, came over as a clown, the better to mask his sharp intelligence as he frustrated endless attempts by corrupt interests to secure global domination.

I came early to the Doctor Who party, apparently watching it from birth early in Hartnell's reign, but my earliest memories are of Pat Troughton, both in the TV Comic/TV21 adaptations and in my classmates and I enacting scenes from The Invasion in the school playground on a Monday.

The difficult job of persuading a family audience that the Doctor could change his physical appearance and personality having been achieved, 1970 saw the introduction of Jon Pertwee… and colour TV! Pertwee's Doctor, deprived of a functioning TARDIS and marooned in late-20th century England, was a dandy, a dashing adventurer in the tradition of the Saint and Adam Adamant, but with a fatal arrogance. For the first time, the Doctor made do with a single companion - the best arrangement, I think - and Jo and then Sarah were two of the finest companions he ever had. These episodes saw the introduction of The Master, Moriarty to the Doctor's Holmes, and the establishment of the Brigadier and his U.N.I.T. cohorts as a permanent supporting cast. They say that everyone has their own Doctor, and Pertwee was my Doctor. I clearly remember the excitement of watching his first appearance, and dissecting each episode with my best friend each week. I never missed an episode of Pertwee's reign, and lapped up the omnibus transmissions broadcast each summer.

The next Doctor was arguably the biggest and best. Tom Baker's louche Bohemian — all teeth and curls — was the definitive Doctor for most of those who caught his unique protrayal, and the length of his tenure ensured that he was the first Doctor for more first-time viewers than any other, topping every "who is your favourite Doctor?" poll (until recently!). Baker brought more energy and more of himself to the role than any of his predecessors. If Pertwee was my Doctor, so was Baker. This period saw some of the most imaginative writing on the show, a broadening of the show's appeal, its highest viewing figures… and its lowest viewing figures. A new producer oversaw a drop in programme quality towards the end of Baker's tenure, when I reached my mid-teenaged years, changing my relationship with the Doctor.

When the BBC miscast Peter Davison, that nice young vet from All Creatures Great and Small, and moved the show to a midweek slot, I became an occasional viewer. Revisiting those episodes is still hard. Davison was just too young, his companions too shrill and numerous, and the new-look console too plasticky, while the adoption of uniforms jarred.

The Trials of a Time Lord

Increasing silliness under John Nathan Turner's reign meant the casting of an extraordinary coat as the sixth Doctor. As memory serves, Colin Baker also appeared in the show around this time. Baker got the short end of the stick. Having joined the show at the end of a season, the disfunctional and disturbing Doctor of that story became the template for his first full season. BBC politics came into play at this point. Successive executives were less than thrilled to be saddled with the show, and scheduled the show against ratings bankers on the ITV network. The extraordinary decision to switch to 45-minute episodes meant a loss of pace. Declining ratings, threadbare visual effects and watchdog concern over levels of violence appeared to make cancellation inevitable, but a highly effective campaign ensured that the show returned after an 18-month hiatus. When the show returned, it was with a short experimental season in which the Doctor, Colin Baker and Doctor Who were on trial for their very survival. Colin Baker went on to become the first Doctor since Hartnell to be sacked from the show, having appeared in the equivalent of just 44 regular-length episodes, not much more than Hartnell or Troughton managed in one year. Having been sacked, Baker, bitterly disappointed, declined to reprise the role for one more story. Another new Doctor then.

While I was of age to regard Jon Pertwee and Tom Baker as my Doctor, Sylvester McCoy was very much my Doctor. It may have been a return to regular, once-a-week episodes of 25 minutes, it may be that I was past my teenaged years and ready to commit to a TV show again, but it was certainly McCoy's beautifully observed performances that nailed these later seasons for me. Script editor and actor were as one again, and McCoy drew on every facet of the Doctor's earlier incarnations, alternately deadly serious or a clown. This Doctor was no longer travelling randomly through time and space but with focus, on a mission, tidying up loose ends. With the arrival of Ace, we had the strongest Doctor-assistant combination since Tom Baker's time. This new sense of purpose brought new excitement and, for the first time since the late 1970s I would wait anxiously for the next episode. Then, disaster. The show was not cancelled, but not renewed. Then it was being "rested". Then it was held out as a possible independent production, or film.

"He's Back — And It's About Time"

Doctor Who lived on, however, as the fan community, now all grown up, wrote novels charting the continuing adventures of Time's Champion. Much excitement followed when, in 1996, Fox Television and BBC Worldwide co-produced what was to be the first of several TV movies with Paul McGann as Doctor Who. McGann's interpretation was interesting — safe, traditional, but quite compelling. Unfortunately, the movie (continuity heavy but ignorant of continuity, overly American, and with a frankly incredible plot resolution) did not work for me, and the show did not pull enough viewers to persuade Fox to commission further episodes. Doctor Who, the world's longest continuously running SF show, was now dead. But that was OK, because we had the novels (revitalised now, with a whole new line featuring McGann's Doctor), and a series of ambitious audio plays by BBC Radio and Big Finish featuring Davison, Colin Baker, McCoy and McGann. With the TV Doctor dead and buried, BBC Worldwide allowed the commission of a new series of webcasts featuring Richard E Grant as the official, new and improved ninth Doctor.

But those pesky fans. One of them became a successful television writer and was somehow able to persuade BBC Wales to commission a revival of Doctor Who. The rest, they say, is television history in the making. It's not the same as it was, but it never was. On air or off air, Doctor Who is constantly evolving. If Doctor Who had not been cancelled in 1989, it would have slowly died a death. The new series is of its time, as well-written and slickly produced as anything else on the BBC's schedules. Chris Eccleston was fantastic (even if Colin Baker and I cannot quite understand why he was allowed to have a Northern accent.) And if Pertwee, Baker and McCoy were my Doctor, well... Tennant is my Doctor.

Forty-five years. Is that all? It's passed so quickly. Here's to Doctor Who. Please, Steven, look after it. We know you will.