I place the blame solely at the door of the so-far lacklustre fourth series for the fact that I somehow managed not to be hugely enthused about the prospect of this Steven Moffat two-parter. And not just any two-parter, but the late series two-parter – which, as we know, have been absolute, stellar, A-grade home run back of the net successes for every one of the three years so far. Whisper it, but as I was at a non-Who-watching friend’s house on Saturday evening, I even entertained the possibility of not watching it live.

It’s a good job they relented, however, as – while they may not have appreciated it – their TV played host to the best forty-five minutes of telly since… well, let’s not beat about the bush, since “Blink”, frankly. Moffat is back, and Doctor Who is back on form – and let’s face it, when it’s on form, it’s the best thing going. Fact.

"What do you see?" "I see some fucking lovely CGI."

Even the pre-titles sequence sparks with more imagination and class than anything else in the fourth series to date: our first glimpse of some utterly beautiful CGI landscaping (and hey, kudos to whoever said “Let’s save our best work for the Moffat eps” – this was a shot to rival last year’s Gallifreyan Citadel), some very Harry Potter-esque but marvellously appropriate Murray Gold music (as with the tech boys, Gold just seems to up his game when he’s working on a quality episode – after all, for all the criticism aimed at him, how amazing was the “Utopia”/”Sound of Drums” soundtrack?) and an intriguing glimpse into the story’s narrative setup (“wait, they’re in her head? What?”). Moffat has such an instinctive grasp of how to structure a show that the way he presents something can be as memorable as what we’re seeing, and so it proves with one of the best openings that he – and indeed Doctor Who as a whole – has given us to date.

The thing that really sets Doctor Who apart in terms of modern British telly is that, above all else, it’s driven by ideas. When the ideas are in short supply, the show suffers. But when they’re flowing, it really is like nothing else you’ll see anywhere – and I include notable and weighty US sci-fi such as Battlestar and Lost in that, as well. It’s about imagination. And Moffat’s episodes are absolutely bursting with the stuff, which is why the prospect of his brain shaping the course of an entire series is so damned mouthwatering. There are more ideas in a 45-minute Moffat ep such as “Silence” than you’d find in a whole series of eps cribbed together by most of the second-tier writers on the series (Messrs. Cornell and Davies are, of course, honourably excepted, being very firmly “top-tier” – indeed, even when RTD’s scripts aren’t the best, you can’t usually fault his flow of imagination).

Take, for example, the “ghosting” process. This is a brilliant bit of techno-projection, worthy of a futurist sci-fi writer like Warren Ellis, not a Saturday teatime kids’ show. And it’s not even the focal point of the episode. It’s a handy device that Moffat has come up with to give pathos to the death of a character we barely knew in one of the most simultaneously moving and chilling scenes that we’ve seen in New Who, and to later instil terror by eerily grafting a dead voice’s repetition to a malevolent skeleton. It’s there to do a job, in other words, and while you might call its application cynical (I prefer to think of it as construction and foresight), you can’t argue with the level of inspiration on show.

Fear, of course, is a big part of Moffat’s stories – the two things he does better than anyone else on Team Cardiff are to play on existing and deep-rooted human fears (in this case, the dark), and to make kids terrified of everyday objects (here, the line about “the dust on sunbeams” practically had me applauding). Since the earliest days of the show, the most talked-about Who moments (outside of hardcore fandom, anyway) have been the ones that scared people shitless, and in four short years Moffat has ensured himself residual fees for nostalgia clip shows for decades to come. He teaches us to take nothing for granted - and that's a valuable lesson whether you're a wide-eyed pre-teen, or a cynical old guffer. Remember that, with the exception of "The Girl In The Fireplace" (whose clockwork robots weren't really the point of the story anyway), Moffat's stories have never given us a visible monster - that's not to say that monsters can't be fun, or even that they can't be scary, but there's something deeply insidious about the fears that Moffat's work teases out, and he's rapidly developing into an absolute master of psychology.

If there’s been a real shame about some of the lacklustre stories this year, it’s that Tennant and Tate haven’t had a huge amount of material with which to spark. It’s led to DT, in particular, starting to look a bit bored with the whole thing – and that’s a shame, because all things considered, I still firmly believe he’s absolutely the best man for the job (there will never, ever be another actor to play this role who will approach it with the same level of enthusiasm. David is one of us, and he will always be loved for that). And Catherine Tate, well… give her her dues, here. Yes, she still has on average one or two moments each episode where she grates, but come on. Did even the harshest of us Tateosceptics ever believe she’d actually turn out to be quite good? The dynamic between the constantly-questioning (and not in a wide-eyed “Doctor, what’s going o-on?” way, but in a forthright and belligerent way) Donna and the Doctor has been, as I mentioned in my “Planet of the Ood” review, perhaps the strongest aspect of this series, and she still comes off like a cross between Sarah Jane and Ace (if less likely to inspire adolescent worship than such a combination might suggest). So much so that I’m almost edging towards disappointed that she’s only around for a few more episodes – and even more so given how poor Freema Agyeman was on her brief return (but let’s not rewrite history, here – she was great in series three).

But here, the pair of them are invigorated once more by the quality of the material. Donna even gets arguably the best line of a script that positively sizzles with zinging dialogue (largely thanks to the presence of Alex Kingston’s sharp-tongued River Song), with her “Ooh, that came out a bit quick!” during the “pretty boy” exchange. The Doctor, meanwhile, bounds around like it’s the early days of his regeneration all over again – without ever slipping into the “AWWH, IT’S SPACE!” or “ooOOWAH YES!” ticks that have become the irritating part of Ten’s legacy.

Meanwhile, the return of Euros Lyn to the directorial helm, for the first time since "The Runaway Bride", is a welcome development. One of the new series’ best directors, he spoke recently of the pressure involved in tackling a Moffat script: “He writes beautiful, intricate, terrifying scripts. Making his dreams live on screen is both a delight and an enormous responsibility.” Thankfully, he’s more than up to the task, and imbues a sufficiently creepy atmosphere to the latter half of the episode, while inspiring a strong sense of awe and wonder in those early library discovery scenes. There’s also a deliberate and satisfying contrast with the “real world” scenes of the girl-slash-security camera, and the clean-yet-garish surroundings of her home feel like a deliberate clue to the nature of her reality – one that’s confirmed by the quite wonderful moment in which Colin Salmon comes across all Morpheus and turns the girl’s world upside down.

In the end, where “Silence in the Library” falls just short of Moffat’s previous episodes is really that it feels like the stringing together of some cracking concepts and set-pieces, rather than a truly compelling story in its own right. Part of this is down to the characterisation – if you’re doing a classic “base under siege” scenario (and “Silence…” is definitely that – Moffat’s first entry into that archetyple Who stable, in fact), then you need a bit more development in the characters than the first episode has so far given us. Mr Lux is the stereotypical arsey leader, the two Daves (nice touch, that – I can’t be the only person who’s noticed how you rarely get two people with the same name in stories, compared with how often it happens in real life) and the American girl are fairly anonymous, and while there’s clearly plenty yet to be uncovered (not literally – this ain’t Moll Flanders) about River in part two, she’s perhaps a little too smug and know-it-ally at the moment. Still, you have to applaud Moffat for another entry into his canon of strong female support characters, even if she’s not yet as brilliant as Nancy, Reinette or Sally; not to mention the way in which he's managed to surreptitiously slip Bernice Summerfield into the series under our noses.

But it’s hard to judge the first half of a story divorced from the context of its partner, and there will doubtless be plenty more to say about “Silence” a week after its first broadcast. Questions about how it holds up as a complete and satisfying story can surely wait until then – for the moment, this stands unquestionably as a cracking piece of television in just about every respect: funny, scary, moving and superbly crafted. All the hallmarks of Moffat’s Who, in other words, and the sense of anticipation for 2010 is surely already at fever pitch. The increasingly tiresome Lawrence Miles has sniped at Moffat in his blog by (among other things) pejoratively using the phrase “new God-King” in the wake of his promotion. I’m fully prepared to use it without a hint of irony.