After a somewhat mixed start, the TARDIS' engines are well and truly revved up on this series now, with excellent stories coming thick and fast. The tenth episode from seasoned Who writer Steven Moffat was always going to be the one to beat, and we already knew it was scary enough to blow the fear factor meter. Right from the first frame the atmosphere of claustrophobic spookiness was established, but nothing could have prepared us for how terrifying the Weeping Angels were going to be, or how quickly they could move when not being looked at. This was "monsters under the bed" stuff at its very best, and all over the country young children will not now be able to walk past apparently innocent statues without a frisson of fear and a long, cold, unblinking stare.

When Sally Sparrow breaks in to a derelict house looking for we know not what, the last thing she expects to find is a message to herself written underneath the peeling wallpaper, dated 1969, telling her to beware the Weeping Angel and also to duck, and signed by the Doctor. She eventually does duck, just in time to avoid a rock thrown into the room from the garden. There's no-one there. Only a statue of an angel which appears to be weeping.

Returning to the house with her friend Kathy Nightingale (excellent bit of dialogue: Sally compares them to a detective duo - Sparrow and Nightingale - to which Kathy replies "it's a bit ITV." Top stuff!) Sally notices that the angel has moved closer to the house. She leaves Kathy behind in the garden and the next time Kathy blinks she wakes up in...1920. Back in 2007 Sally answers the doorbell to Kathy's grandson who hands her a note explaining what happened to Kathy and that she's lived a full and happy life with children and grandchildren, but has since passed away.

Sally doesn't believe it, and chases all over the house looking for Kathy. She finds Weeping Angels in various rooms, one of which is holding a key which Sally takes. At the request of Kathy's note Sally goes to pass on a message from Kathy to her brother Larry who works in a DVD store. Larry has been investigating an "Easter Egg" in a series of DVDs that no-one, not even the manufacturers, seems to know anything about. The Easter Egg is a short video clip of the Doctor apparently holding a one-sided conversation. He seems to advise Sally to tell the police about Kathy's disappearance. At the police station, DI Billy Shipton shows Sally a collection of abandoned vehicles, all of whose owners have parked up at the deserted house and disappeared. One of the "vehicles" is the TARDIS. In the blink of an eye the TARDIS is surrounded by angels and once Sally leaves, DI Shipton blinks and finds himself in 1969. He meets the Doctor, who gives him another message for Sally. The Doctor also explains about the angels - an ancient race who can only move when not being observed by another species and who live on the potential energy from the lives of people they have sent back into the past. "The most gentle killers in the universe," the Doctor observes. "They transport you to the past and let you live yourself to death." Once again, utterly brilliant dialogue.

Back in 2007 Sally realises the key she has must fit the TARDIS but by the time she returns to the police station it is gone. Billy calls her and asks her to visit him in hospital. Having lived since 1969 he's now on his death bed. Bitterly regretting his lost chance with Sally, Billy passes on his message about the Doctor in the DVDs and Sally stays with him until he dies.

Sally and Larry play the full DVD message. As Sally listens, she responds to the Doctor and the conversation is revealed to have two sides. Larry starts to write it all down, expecting to impress his friends on YouTube, and the Doctor explains that it's Larry's transcript that is allowing him to hold the conversation. The Doctor tells Sally she must ensure the Angels don't get hold of the TARDIS, which would provide them with enough time energy to take over the universe. Sally and Larry manage to escape from the Angels and enter the TARDIS, where one of the DVDs allows them a "one time" journey. Only it looks as though the Doctor has tricked them - the TARDIS starts to dematerialise leaving Sally and Larry behind and at the mercy of the Angels - they start to attack through the walls of the departing TARDIS. But the trick is on them. As the TARDIS leaves the Angels look at each other, and each therefore immobilises the others - forever.

A year later Sally and Larry have set up shop together when the Doctor passes by on another mission. With shades of The Time Traveller's Wife, Sally rushes out of the shop to hand over the transcript of the DVD "conversation" and is initially surprised when the Doctor doesn't recognise her. But she soon cottons on and wishes the Doctor luck with his adventure.

I wasn't joking when I said hundreds of young people will be spooked by statues for weeks to come. Hell I'll be giving them a wide berth myself. Those Angels were very scary when they were eventually caught on the attack, and the editing that made them appear to move so quickly was first-rate. This is the first "modern series" episode to have the Doctor take a back seat dramatically (enforced by the shooting schedule, apparently) but the story definitely didn't suffer for it. I would have loved to see this episode given room to breathe as a two-parter. The extended format gave enhanced life to the Family of Blood story and I'm sure this too would have been even more powerful if we'd had time to build up the tension of the moving Angels over a longer period. Nevertheless it was a fine episode, well up to Moffat's self-imposed standard and one of the strongest so far this series, along with the opener and the aforementioned Family of Blood.

For a series of stories about a time traveller, Doctor Who rarely wanders into the realms of ontological paradox (or any other kind of paradox for that matter). I suppose a little of this goes a long way, but even so it's very welcome when it comes and was superbly handled in this story.

Next week: Utopia.