Silence In The Library finds writer Steven Moffat in an expansive mood. Combining his love of occult iconography and gothic sensibility with the broader populist style of Russell T. Davies is a move that may offend Moffat purists - who are used to his episodes being one-off excursions into the dark side - but it gives us all a realistic taste of series 5…and it tastes pretty good.

The wonderfully eccentric idea of a ‘library planet’, where the biography wing alone occupies a continent, could be seen either as a cheap way of shoe-horning a spooky setting into a science-fiction show in need of greater spectacle, or as a delightful Douglas Adams-style blast of invention, and in fact it works either way.

The episode opens with a little girl in some kind of retro-present-future environment (the phones were out of fashion even in the late 70s) apparently undergoing psychotherapy with moody shrink Colin Salmon, and dreaming of floating over the astonishingly well-realised cityscape of the abandoned, 51^st-century library-planet.

But the planet is not make-believe; the Doctor and Donna are there too, along with something unspeakably dark and very hungry…

New Doctor Who villain/monster the Vashta Nerada constitute Moffat’s sci-fi explanation of a (apparently universe-wide) fear of the dark: “piranhas of the air – shadows that melt the flesh”…a swarm-race of microscopic creatures that imitate shadows in order to creep up on their prey, an ingeniously creepy idea.

The scene where our time-travellers are running from a darkening corridor is chilling, and Library goes on to layer intrigue after intrigue: a small band of hi-tech archaeologists have spent three generations trying to gain access to the planet, which was automatically locked when the Vashta Nerada wiped out the entire population a hundred years earlier. When they burst in and take their helmets off, leader Professor River Song (Alex Kingston) seems to have all the accoutrements of a time-traveller herself, from the sonic screwdriver to memories of the Doctor’s personal future.

I'm not falling for the debatable romantic interpretation of Kingston's affection for our Doc, or for Donna's jealous looks. If this isn't the regenerated ‘Jenny’, the ‘doctor’s daughter’ from a couple of episodes back, I will buy a hat and eat it. It’ll have to be a chocolate hat though – part of the delight of Moffat’s writing lies in the surprises.

A ‘Donna-moment’ seems to be requisite for each episode now, as Catherine Tate has truly become the sensitive side of the Doctor, a configuration that goes back to the Hartnell days. Here her tender moment is with the spectacularly dumb and spectacularly doomed Miss Evangelista, a glorified secretary and passenger in the archaeological expedition who is self-conscious about her IQ and unregarded by her colleagues.

Donna takes pity on her whilst the mad girl in some other world is using hidden buttons on a remote control to unwittingly fling books all over the library, poltergeist-style.

Proving her stupidity – and an old-style horror movie convention – Miss Evangelista is dumb enough to go down a newly opened hallway that only she has noticed, only to be instantly pared down to a skeleton by the Vashta.

The following scene, where Evangelista’s ‘data-ghost’ (a semi-intelligent messaging system that persists for a little while after death) suffers a poignant parting from Donna demonstrates, I believe, the increased cohesion of the strands in series 4: the computer-records on the library-planet declare that 4022 people were saved in that building at the time of the disaster, but that there were 'no survivors'. Could next week’s Forest Of The Dead consist of the data-ghosts of those unlucky library-dwellers?

The automated librarian-totems with real (donated!) faces are a deliciously gruesome little invention, but even that is not thrown away, as Donna is to become one of them by episode’s end.

With so many unanswered questions, any ultimate judgement on the story must be deferred to Forest Of The Dead. The only bit I had trouble with was the notion of a swarm of insects getting together enough muscular cohesion to make the suit of their latest victim walk, zombie-like, towards the surviving protagonists in a great end-of-episode cliff-hanger.

If Kingston is not the Doctor’s daughter, who is she? Are all the characters in Library living their reality inside some kind of program under the aegis of the little girl? Is the girl herself an intelligent program that has devised her faux-reality in an effort to resolve a conflict created by the global catastrophe that has emptied a planet-sized library? Has Professor River Song’s dark look about Donna’s future already manifested in Donna's grisly transformation into a glorified speak-your-weight machine? There’s clearly more to Colin Salmon’s child-psychologist than meets the eye (seeing as he tells his patient that her dreams are real and her reality false), so who will he turn out to be?

I’m glad I care. It’s been a while.

It’s not actually Steven Moffat’s four self-penned episodes in series 5 that I am most looking forward to – it’s his rewrites of others’ scripts. In Library, the usual fast-forward-style dialogue is slowed to something that you can actually follow, while the pacing is expert and the humour issues far more from situation than self-parody. Nor can the improved pacing be ascribed to the extra-length given to the story (and I am guessing there may be fewer one-off stories under Moffat, who clearly loves cliff-hangers), as evidenced by the breakneck speed of the recent Sontaran two-parter.

Donna has one ‘Catherine Tate’ moment, sawing those planks again with that raucous voice in a moment of anger, but I will have to get used to this kind of thing in 2010, when Moffat starts his very difficult balancing act between the past and the future of new Who.

The SFX and production design in Silence In The Library were unfaultable, and the initial fly-by of the dead cityscape could comfortably stand amongst the very best work of ILM or Weta Digital. Thus Library constitutes a challenger to Fires Of Pompeii for sheer effort (and presumably money) expended in realising the unexpected environments of Doctor Who. There are many delightful little retro touches in the episode, such as the floating security globe through which the mysterious girl interfaces with the dead city, which seems to be made of mahogany.

The library itself, apart from being genuinely scary, is a fantastic achievement for any television series. Logic tells me that many of the pans past endless dusty corridors are cloned from one instance, but it’s all seamless.

Director Euros Lynn needn’t have worried too much about doing justice to Moffat’s work – the cinematography, sound and acting were all first class, Tennant toned down his shouting and Billie Piper was not in evidence (yet). Is it too much to hope that she crawled into a nice shady nook thereabouts for a nap?

Silence In The Library guest star Colin Salmon had a chat with DoG not long ago.

Check out our reviews of the previous episode, The Unicorn And The Wasp. Simon's is here, and Martin's is here.