"Flawed masterpiece". That's how Tom described S.T.A.L.K.E.R.: Shadow of Chernobyl in his review last year. Seems like Ukrainian developer GSC Game World took notice, because it's not doing a proper sequel right now. Instead it's doing a prequel, called Clear Sky, and, according to the company's PR director Oleg Yavorsky, it's all about making things right.

"There were a few things that were new about S.T.A.L.K.E.R.," he says. "The first was this concept of artefacts, stalkers, anomalies, we had the Chernobyl zone. We felt that we needed another explanation to the players of these things, how they work, before we do something big next like a sequel story.

"Secondly there was this Strelok guy who was the main character from the original game and his story was also strange and not very well explained. Through the course of the game you find out that he made several trips to the zone centre and as you play it's his third campaign, but what happened during his first two visits? You don't know. So this is something we wanted to explain with a prequel and at the same time give an alternative look into Strelok and his doings there so players get an understanding of the place and what's going on."

Strelok's an interesting one, because this time instead of playing as him you'll be going up against him as a mercenary. The game is set once again in the Zone, the site of the fictitious second Chernobyl explosion S.T.A.L.K.E.R.'s story centres on. There are 12 locations in total. Half of those will be totally new, the rest will be locations from the first game given a sprinkle of magic graphics dust and redesigned to fit the new story.

Clear Sky still focuses heavily on open world MMO style FPS action. From what we saw of Oleg's live demo of the game, speaking to NPCs, gathering quests and venturing out into the wild will form much of what you'll be doing in the game. The difference this time however is that instead of following a somewhat linear plot you'll be able to interact with up to five factions, one of which is called Clear Sky. (Oleg reckons Clear Sky packs in about 20 hours of gameplay if you concentrate on the main story only, and plenty more if you explore the open world).

Character models have been improved

The factions are much more developed this time around. Each faction will have a fully fledged base with a leader, trader, barman, mechanic, guides and a tactician. These bases will be the main hub from which you pick up quests. Oleg shows us the Clear Sky base, a simple, run down set of structures. Around camp fires people sit and sing, play musical instruments and share anecdotes. And yes, vodka drinking is in the game.

Clear Sky are scientists that were part of the experiments that led to the creation of the Zone. They believe that scientific study will help find a solution to the chaos. They reckon the Zone has reacted to something that has entered its core and the prime suspect is Strelok himself. Clear Sky want him stopped before the Zone blows out uncontrollably. This is your primary mission throughout the game.

In the last game, says Oleg, money was easy to earn but there wasn't much to spend it on. Clear Sky will have a much greater incentive to spend cash on upgrading your weapons from a redesigned inventory screen. You'll be able to create up to 16 upgrades per item. Oleg shows us the Clear Sky mechanic, an NPC modelled after an actual mechanic from the real Chernobyl Power Plant. "A friend of ours" he says.

You'll once again be wandering about in the Zone

Once again anomalies will play an important part in gameplay and once again you'll be hunting down artefacts using a Geiger Counter in one hand and a pack of bolts in the other. More anomalies have been added to the environment. We see a strange pool of whirling water sitting in the middle of a calm stream. We're promised odd ruins, distortions in the ground and teleporting portals too. "We wanted to create the impression of the Zone as more dangerous and hostile," Oleg explains. He shows us how anomalies will work in Clear Sky, part of the game's opening tutorial. With a set of bolts in one hand and a detector in the other he throws the metal and listens to the clicks to work out where it's centred. Eventually the artefact will reveal itself. "Here's where your protective set upgrades will be useful. Some you won't be able to enter until you've upgraded your equipment to a high level."

The tutorial continues. Oleg hides from dog/pig/rhino monsters by climbing a tower. He starts taking pot shots at random birds flying overhead. In Clear Sky you'll be able to shoot them down. Bandits will have contests in dedicated arenas which you'll be able to take part in. "Greenpeace hate!" he says. We agree.

Blow outs are not a much bigger part of the game, which, according to Oleg, was what the team had always intended for the first S.T.A.L.K.E.R. Although the main character can resist the blow outs, his nervous system will still break down. The sky goes blood red. You're told to hide. Eventually it overcomes you and the Clear Sky faction come to your aid, bringing you back to the base and ending the tutorial.

We then take our first tentative steps into the outside open world and the Marshland level. The area is littered with four-legged monsters which Oleg dispatches with the shotgun. Then there are bandits to contend with. But it's the faction on faction combat which will impress gamers the most. The Zone is embroiled in a constant AI driven conflict as the factions battle for territory, resources and scientific information. At any time the player will be able to check which faction is winning the war in a separate War of Factions screen, and at any time the player can choose an ally and get their hands dirty. The more points controlled in the Zone the more resources are gathered and the more quests and equipment becomes available. "It changes dynamically," enthuses Oleg.

We then see the squad-based gameplay in action. Oleg accepts a quest and a squad forms then makes its way out into the wild. His AI buddies crouch as they move by low cover, and even crawl if they have to. They form positions, poking their weapons out above them and firing blindly. So smart is the AI that your computer controlled mates will throw grenades.

So, Clear Sky is more S.T.A.L.K.E.R. 1.5 than 2.0. That's absolutely fine with us. But don't let that fool you into thinking GSC has simply tweaked the menus, given the AI some smart juice and drummed up a new story. Clear Sky is S.T.A.L.K.E.R.'s graphics on steroids. It's all down to the new DirectX 10 support. New textures and new high poly models have led to an increase in detail not only in the characters but in the environments, too. In a tech demo we see volumetric fog, which changes and warps when characters move through it, leaving trails. Rain dynamically wets surfaces. At first the ground will glisten, then puddles will form with droplets bouncing up and down. Water streams down vertical surfaces realistically.

NPCs act naturally, whether in combat or not.

But we were most blown away by the lighting, which is some of the best we've seen in a game. It's been completely changed. Camp fires cast dancing shadows that impact on walls and characters in real time. Sun rays burst through cracks in wooden roofs. Lightning lights up the night sky as well as dark interiors. It looks superbly atmospheric, which is what S.T.A.L.K.E.R. is all about really.

Don't worry, the game doesn't force you to own a £4,000 super computer just to get it out of bed. Clear Sky will support three ranges of hardware, DirectX8 through 10. It will run fine if your computer coped with the original game.

We're very enthusiastic about Clear Sky. It's shaping up to be a graphical PC powerhouse, and a worthy refinement of the original S.T.A.L.K.E.R. vision. It's surely one for the fans - don't expect to like it if you didn't like the first game. But that's OK. Because we were fans anyway.

We also expect it to hit its planned August 29 release date, so there's just under three months to wait. The team is putting English localisation in right now, so soon enough we should have some brand new playable code to explore. Until then, S.T.A.L.K.E.R. remains one of 2008's most promising PC exclusives.