It would be easy to feel cheated. You play a game called "Stalker", and there's not a single celebrity in sight... Then you remember that the game is actually called "S.T.A.L.K.E.R." and that it isn't about violating a restraining order and tailing A-list movie stars down Hollywood Boulevard in the Californian sun, but instead there's some nonsense about a reverse-engineered acronym and you're spending your time hunting alien artefacts and trying not to be eaten by mutants in a Ukrainian thunderstorm.

Clear Sky is the prequel to the original S.T.A.L.K.E.R. game, Shadow of Chernobyl, and begins with your character being caught up in a mysterious emission of monumental proportions originating from the centre of The Zone - a region of radioactive and psychic instability created in wake of the explosion of Reactor 4 at the Chernobyl nuclear power station. You awake after the emission in the secret base of a Stalker faction called, somewhat predictably, Clear Sky, a group who are conducting research on the nature of The Zone, who co-opt you into aiding their cause in exchange for information that will send you on a quest to put a stop to the catastrophic Zone emissions, which are slowly destroying your nervous system. Clear Sky are one of six competing factions in the game, two of which (the Renegades and the Bandits) are hostile, while the others will call on your aid throughout the game as you uncover further details as to the cause of the disruptive emissions that are killing you.

If you played Shadow of Chernobyl, Clear Sky's Zone will strike you as a much wilder, unforgiving place by comparison. There is a greater variety of anomalies (supernatural phenomena that can do anything from blasting you with a column of fire to dealing out huge doses of radiation), all of which are potentially lethal if you linger around them too long. The artefacts they contain are likewise harder to acquire, though no less worth the effort and risk. To aid you in your search for these valuable alien devices, you are provided with a supply of metal bolts to use as a manual sonar device, to help you plot the boundaries of an anomaly and prevent yourself from blundering into a quick, messy death. It's a useful addition, but it's not adequately explained how to use them in game, necessitating a thorough scouring of the manual before you can use them, unless you resort to random button-pushing.

In general, Clear Sky would have definitely benefited from a tutorial, especially one that explained both the finer points of plundering anomalies and the less obvious aspects of the user interface. The game assumes a lot of prior knowledge, so if you happen to be coming to the series fresh, you're going to die a lot, because the interface is quite subtle about giving you feedback when you're bleeding to death because you haven't bandaged a gunshot wound or when you're suffering from radiation poisoning.

Clear Sky's Zone might be unforgiving, but it's certainly more eye-catching than in Shadow of Chernobyl; you could even go so far as calling it pretty (at least as far as radioactive wastelands go). The dynamic lighting effects are little short of spectacular, but do come at a price. Even after the first post-release patch, you can expect to take a significant performance hit while running the dynamic lighting effects, even well beyond the recommended specification. Static lighting yields much better frame rates, and does not really make a huge amount of difference to the game experience. The revamped graphics engine really adds to the atmosphere of the game. Exploring mutant-infested landscapes after dark with only a head-mounted torch to light your way is creepier than a box full of centipedes, and you will be twitching at the sound of every rustle in the undergrowth, wondering if there's a mutant boar or a pack of blind dogs lying in wait for you in the shadows.

Indeed, combat will keep you on your toes in general, because it's punishingly hard, even downright frustrating at times, even at low difficulty levels. It's reminiscent of Operation Flashpoint in terms of the player's vulnerability, since the AI is no pushover and is very fond of using grenades, which are an instant kill unless you can evade the blast radius in the short time between the HUD warning that they've been thrown and the time they explode. Clear Sky, therefore, rewards a smart fighter, rather than one who goes straight for the jugular, as direct confrontation with multiple enemies is rarely survivable, at least until you begin to acquire assault weapons. Like Operation Flashpoint, the combat has a fairly steep learning curve, but the sense of achievement and reward for being able to finesse, rather than bludgeon, your way through encounters is very satisfying and rewarding.

It's an improvement on the first game, but still quite unpolished.

Less impressive, however, is the implementation of other aspects of the game design. As a mercenary, your character will be called upon to assist various factions with tasks - such as defending a camp from mutant attack, or repelling a military incursion into Stalker territory - but the way these tasks are generated means that you will often be asked to perform the same task repetitively, sometimes before you've even collected the reward from the previous occasion you completed the task. As annoying as this might be, however, other decisions boggle the mind. While the narrative of the game is more direct and engaging than its predecessor, and all the better for it, the enforced mugging by the Bandit faction the first time you enter The Garbage map seems to serve no other purpose but to frustrate and delay the player, by stripping them of all the money they have accumulated to date. This kind of event was annoying back when they released Half-Life - now it is unforgivable, particularly if you've been hoarding all the cash you've gained from quest rewards to buy a high-end set of armour, only to have it stolen away with no way of getting it back.

Equally annoying is the way the game herds you towards making your initial foray into The Cordon (the starting map in the original game, incidentally) by passing through a military encampment packed with psychic guards with ultra-accurate machine guns. And if you happen to be making the trip at night, the chance you have of surviving the encounter is precisely zero... The alternative is to fight your way through two camps of hostile Stalkers on two separate maps, though that actually turns out to be the easier option. The game is also still quite buggy despite the patch, with numerous glitches still in evidence, varying from the mildly confusing (inaccurate calculations of remaining ammunition) to the very annoying (not being able to switch ammo types for seemingly no reason at all).

While Clear Sky's myriad of minor bugs and flaws may irk, they do not render the game unplayable, so while things could have ultimately been a lot more polished and well considered in execution, there's still a lot to like. The more personal focus of the narrative gives the game a more concrete direction and drive, while still allowing the player freedom to roam away from the main quest. The graphical improvements help build the game's atmosphere and sense of immersion, and the upgrade system for your weapons and armour give the player greater freedom to customise the main character according to their play style. As a prologue to Shadow of Chernobyl, Clear Sky undoubtedly fulfils its remit, and in many respects is the superior game, but a further few months of polishing and honing could have banished that faintly damning tag "flawed masterpiece" entirely.