S.T.A.L.K.E.R.: Clear Sky Preview

War is like Christmas for some developers. It has ready-made stories, clear-cut opponents, and there's normally even a party at the end. War's also got the classy sheen of history, but it's history reduced to the bouncy bits: a chance to look sober while savouring the massive explosions. Not many human tragedies let you do that.

Nuclear disasters have massive explosions, too, but so far the real life examples haven't been such a good fit for videogames. Science-fiction may love a post-apocalyptic world, but it doesn't want reality stepping in to humanise the cliché. Nobody logs onto Xbox Live of an evening for a quick round of Three Mile Island, and so far there's only been one game of note dealing with a historical meltdown.

Well, duck and cover: it's getting a sequel. 2007's S.T.A.L.K.E.R.: Shadow of Chernobyl was always a wilfully difficult beast - at a time when most FPSes found it hard standing out, GSC Game World's title had trouble fitting in. Taking cues from Arkady and Boris Strugatsky's classic novel Roadside Picnic, Shadow cast the player as a scavenger in the exclusion zone around the ruined power plant - a fictionalised world home only to madmen, paranoid factions, and radioactive treasures. Appropriately, there was something mutant about the game itself, as it attempted to splice sandbox exploration with a linear plot, and evolve RPG upgrade elements out of an FPS's body.

The result was a game that was impossible to forget, but equally hard to classify. Ultimately, Shadow could never quite decide whether it was a story or a place. Those expecting the forthcoming prequel to clarify things can give up now. If anything, S.T.A.L.K.E.R.'s about to get even more confusing: the developers are adding yet another big idea to the mix.

Clear Sky will feature new varieties of anomaly: some can now work as teleports.

Sitting down to watch S.T.A.L.K.E.R.: Clear Sky being played reveals that new idea is to play up the role of the factions. Present in the first game, and part of the much-vaunted ALife artificial intelligence system, originally designed to create a believable ecology of NPCs who would travel through the zone independently of the player, the factions stuck out as promising ideas awaiting the correct implementation. This time, they're centre-stage, and the aim, at least for the majority of the game, seems to be choosing between them before getting involved in their battle for supremacy.

Simplistically, this means that each faction has its own base somewhere in the exclusion zone, which acts as a hub for missions and side-quests. But on a wider level, the faction mechanic plays out almost like a ground-level RTS, as different groups fight for control points, giving them a greater grip on resources, territories and scientific information.

The weather effects benefit from DirectX 10 gloss. We'd definitely suggest packing a parka - preferably lined with lead.

In gameplay terms, this currently would seem to cement the original game's eccentricities rather than curing them: Shadow was always more competent at making you stumble across activities than it was handing out well-structured missions, and as our preview of Clear Sky progresses, almost every story-focused goal that gets started ends up creatively derailed by another distraction, with the fight for territory top of the list. It's hard to tell how much of this is down to the state of the demo build (it could even be that our demoer is avoiding the main plot due to the untranslated Russian text of the current version) but the effect is as disorientating - and refreshing - as it was first time around.

Clear Sky has twelve areas - six are entirely new, and six are familiar, but significantly reworked. A visit to Escape, a woodland area with hills and bunkers which cropped up in Shadow, shows just what that means. The lay of the land, if not the landscape itself, is very different this time around - a Stalker-held area rather than the military stronghold of the first game.

Looking for missions at the nearest base, it's not long before we're caught up in a control point firefight playing out over a section of highway. As we temporarily put narrative concerns to one side for the first - but certainly not last - time today, we get a chance to see how S.T.A.L.K.E.R.'s weapons have been tweaked. A common complaint of the original game was that it was easy to get money, but there wasn't much to spend it on. Now each of the game's determinedly unexotic arsenal of hulking shotguns and weighty pistols can have up to sixteen possible levels of upgrade added. Modifications are performed by faction mechanics, and each addition comes at a cost - for example, adding sniper scopes to a gun might mean you won't be able to improve its rate of fire further down the line.

The control-point battles are also a chance to see ALife flourishing: skirmishes seem to break out organically, and even when the fight's over, there's every chance the loser will eventually come back with reinforcements. To aid this, Clear Sky is tentatively squad-based: AI team mates can be given basic commands, but will also be capable of finding cover and looking after themselves. At the moment, they may be too capable: as our developer takes his hand of the mouse to explain the cover system, his team quickly finish off the opposition without him. Players can still choose to go it alone if they miss the lone wolf feel of Shadow, but Clear Sky will reward you for keeping company, and GSC Game World's past form with AI suggests squads will be fun to have around.

Although specific effects are DirectX 10-reliant, the game scales to DX 8 and 9 machines.

Moving on to a brand new area reveals further opportunities for distraction. Marsh is a stretch of waterlogged sandbanks dotted with sniper towers. Trees sprout from the ground in hesitant clumps, and the odd rusting hulk of a building lingers in the distance, but this is primarily a wild area, filled with mutant boars and bandits. The map is vast, and we've objectives to complete here, but almost immediately, we're off-track and investigating anomalies - areas with unpredictable physics, which now play out like clusters of cosmic landmines hiding artefacts, the game's loot. With upgradeable artefact-detectors available, and an almost mini-game-like maze mechanic to reach the loot, anomalies seem likely to provide another entertaining time-sink.

It's almost a relief to find that S.T.A.L.K.E.R. remains as idiosyncratic as ever. Far from a more commercial sequel, at the moment, Clear Sky seems like a real curiosity. A single-player MMO or a radioactive Animal Crossing, it excels at providing activities for you to do, but may still struggle when it has to focus on a narrative. The team's overall solution doesn't really convince, either: the plan is for the game to be loosely divided into halves, the first section being open-ended and explorative, and presumably holding the majority of the faction activity, before giving way to a more controlled and linear second part where the story takes over. It's a risk - Assassin's Creed stumbled in its later moments with exactly the same move - but it may be the only way to get the balance of story and environment that the developers are after.

AI team mates are smart, but they haven't worked out how to dress to flatter their body shape.

What's easier to see is that Clear Sky is a lot more beautiful than its predecessor, with a wealth of DirectX 10 effects such as volumetric smoke and dynamic shadows. More importantly, however, Clear Sky also inherits Shadow's melancholic art direction: a landscape of folkish campfires, tumbledown water towers, and crippled powerlines.

Those were the touches that made the abrupt and baffling world of Chernobyl so interesting the first time around, and after a few minutes of watching our developer wander the shantytowns of the Zone, it seems we've never been away. There's still a lot that Clear Sky can fail to capitalise on, and the potential for confusion is greater than ever, but GSC Game World has clearly stayed true to its homely mutant - if anything, it looks like it's continuing to evolve.