What separates the good horror films from the really good ones? What holds the genuine shockers apart from the moderate spine-tinglers? I've been thinking about this - for at least two minutes - and I'm pretty sure the answer isn't sharks. In fact, I'm pretty sure the answer is something like ambiguity: a creepy sense of uncertainty, a little bit of leeway for your imagination to work its dark magic.

And sharks, of course. But Silent Hill: Downpour doesn't have sharks in it so let's stick with the ambiguity for now.

Silent Hill's always thrived on ambiguity. It offers you passage into a half-seen world, obscured by mist and rust: a place where abandoned buildings poke out of the fog as you creep closer, and where two conflicting dimensions grind against each other in the shadows, one a landscape of faded fifties Americana, another an ugly and corroding force of entropy.

Downpour's got all of that, of course, but it also takes the ambiguity a little further. This time, you've been dropped into the shoes of Murphy Pendleton, who sounds, now that I think of it, like a brand of open-toed sandal ("Barbecue, darling? I'll bust out my Murphy Pendletons,") but is actually a prisoner being transported to the slammer. Brilliantly, the game's early moments leave you with few clues about why Murphy's been sent down, and that makes for an experience where you're eagerly waiting for clues to your own character as well as to the mysteries of the environment.

The opening cinematic of the game's latest short but fiercely atmospheric demo build works wonders with this notion. Murphy's prison bus crashes on a lonely stretch of highway and we're left with no idea as to whether it was an accident or something far worse. Then, in the blur of action that follows, a state trooper falls to her death from a cliff, and it's hard to tell whether Murphy was trying to save her or was giving her a parting shove.

Video: Casual dinner party? Don't forget to polish your Murphy Pendletons!

As Murphy heads into town under your control - uh oh, the town in question is Silent Hill! - you can't help but read things into his appearance and his demeanour. He has a thin, rattish kind of face, like Denis Leary, which can't be good, and he pads through the game slowly, yet heavily, his frame bent forward, as if he has the spirit of a predator perhaps, or the hip joints of a really old lady.

Once he's in town, Murphy's the least of your problems, however. Actually, he's the least of Murphy's problems, too. Vatra Games, the Czech studio handling the latest Silent Hill, has no problem pouring on the derelict backwoods ambience, and things are soon nice and creepy.

A gas station looms eerily out of the mist as you round a corner. There's nobody there, but when you smash open a nearby gate with a crowbar and work your way onto the roof - never look for reason in this sort of game, it ruins all the scares - you discover a wheelchair abandoned on top of a pile of pressed tin plating - and its back wheel is still turning!

That's nothing, though. Wander onwards and you'll find a cable car that will take you deeper into town. The only problem is that the cable car is operated by tokens, and the token machine has been stolen.

A puzzle has clearly appeared, and while it's a simple variation on locked doors - and only a little more evolved than the locked door you had to smash open a few minutes ago - the detailing is wonderfully disquieting. Whoever took the token machine had to drag it behind them as they went, and it's up to you to follow the tracks out the back of the building, across a leafy stretch of deserted highway, and into what can only be described as the world's nastiest diner - and that's taking into account a place called Norm's that Sega once took me to in Santa Ana. Things don't start well at the Devil's Pitstop, as this establishment is called: after a quick chat with a friendly Silent Hill postman who turns out to be a ghost of the vanishes-when-your-back-is-turned variety, you head inside only to find yourself plunged into almost total darkness.

Downpour's lighting - or rather its lack of lighting - is beautifully handled, forcing you to head deeper into locations you'd really rather not go near at all, just in order to simply orient yourself. In the gloom of the diner, all busted tiles and smeary windows, you'll have to find your way to the kitchen using only the glow from a nearby jukebox: luckily there are chairs scattered around along the way for you to arm yourself with and, besides, it's hard to be miserable near a jukebox.

Once you're in the kitchen, those Czech designers reach for the slider marked "trippy". (It's next to the slider marked "sharks".) There's been a gas leak, by the smell of things, and when Murphy goes to turn off the ovens, the whole place bursts into flame. A kitchen fire in a deserted diner would probably be terrifying enough for most of us - particularly since the place is probably insulated with, like, asbestos or something - but Silent Hill's just getting, um, warmed up. First the blaze turns on the sprinkler system, and the room starts to flood - water is the game's dominant theme, incidentally, and it also happens to be Murphy Pendleton's greatest fear - and then the landscape starts to collapse inwards as that famous Silent Hill entropy effect takes hold.

For the last few minutes of the demo, Murphy's lost and terrified in a grisly dimension of rusting metal and mouldering horror: a place where half-eaten corpses are bound to bed frames by bonds of barbed wire, and where something unspeakable is intent on pursuing him as well as on acting like a tutorial to show you that the left bumper activates a kind of rear-view-mirror cam. Murphy's off and running, then, as you lead him down endlessly extending corridors and over sudden steep drops. Then, as quickly as it started, it's over.

As demos go, it's short and gorgeously incoherent, but it's enough to showcase a game that's being put together with a real feel for authentic scares. We'll need a longer glimpse of this to see how Murphy shapes up - and to see whether the developers' ambition to bring a more open-ended approach to puzzles has paid off - but for now, the sense is of a game that's filled with plenty of questions and plenty of sprightly shocks. And all that without a genuine horror game enemy in sight. Or a shark, come to think of it.