Who'd have thought we'd ever be in a situation to say that a Doctor Who story will now forever be remembered as "the one with the farting"? You can feel the love for RTD ebbing ever so slightly away as the impact of the scenes - not merely one scene, a moment that can be forgotten, but a galloping and impossible-to-ignore sequence of them - sinks in.

It's bizarre, if you think about it. Not entirely uncharacteristic in a sense, since we knew this new incarnation of Doctor Who would be aimed squarely at the kids and stuff us if we didn't like it, but still reckless in the sense of your well-mannered librarian Uncle suddenly becoming an airline pilot. Is this the Doctor Who series' early mid-life crisis?

This flippant, ten seconds of "look no hands!" while at the helm of the Who Mercedes, a disaster in the history of television, is worrying not really because of what it is - flatulency in Prime Time - but because it betrays RTD taking an unusual foot of the genius pedal. Over the past few years, he's nicked something off Doctor Who fans which previously many combined (Dapol, Longleat, the BBC) had exhausted our supply of - trust. Yes, whisper it but the run up to this new series has been a heady series of reassurances, grumbling doubts, and blow-me-RTD-was-right's. We questioned if it could come back, if it could work without cliffhangers, if the Doctor could still be good without looking gay. And each time, it DID work. Ask the neighbours, and more importantly Little Mr and Little Miss Kiddy neighbour, because they love it. We were in WHSmiths on Saturday and were struck down and overcome by two small boys playing near the DVD section. One seemed to be stroking the cover of the "Leisure Hive" and murmuring the word "alien!" while gazing at a Foamasi. Then his little friend noticed his curly locks and pointed out that he looked like Tom Baker! There it was, eight year olds who knew who Tom Baker was again! Was it 1975 once more? We loitered by the playing children attempting to lap up their every word of adoringness for our creaky old show until security escorted us away, whence we could only marvel that RTD really had brought Doctor Who home to the masses and - yike! - the Next Generation. We trust him, now. Or did do, until the farting.

There was surely no in depth reason for lots of characters to break wind on-screen and bring our cherished show crashing down to serious Dick and Dom levels. There's going to be no "ah that explains it all!" this time. What exactly are "gas exchanges" anyway, and why do you need them to occupy a human beings skin? A low burbling, maybe even a not unfart-like squelching, would have given us a get out clause. But, no, this was wind breaking of public school boy proportions. Proper, joyous, earth-raising guffing. We even got the phrase "silent but deadly" used on-screen! In fact it's a mystery why Davies didn't note in one of his wretched columns last year that "episode 4 contains the word farting..." - we could have got all this hysteria out the way months ago. It's at this point that we begin to see the separation between adult humour that kids can enjoy, and kids humour that anyone other than a kid finds childish. If only there'd just been a half-decent reason for it.

So why did he do it? My other half's guess is someone venturing an outlandish "what if..." during an especially creative (and possibly wine-fuelled) writing session. Or perhaps it was the ultimate stand against the fact that we were happy that he hadn't DONE anything he shouldn't (we've even got used to that funny TARDIS). We've had trumping in Doctor Who. They can do anything now and we can't mind more.

Or perhaps it's all explained in, you know, that half of the story that none of us have seen yet? If it is, I've just wasted a lot of column space and some of you might want to eat humble pie. Or feel unchanged that even for one night we didn't really need it. Fat people farting in Who. No thanks.