The Zone isn't a friendly place, and if you played last year's shooter/horror/role-playing hybrid S.T.A.L.K.E.R.: Shadow of Chernobyl, this comes as no shock. Clear Sky offers a few improvements and a number of issues, but the real star of the series--the large, barren wasteland created by a fictional explosion of the infamous nuclear facility in Chernobyl, Ukraine--is still the most impressive aspect of this prequel. This isn't a full-fledged follow-up, but rather a stand-alone expansion set before the events of the original. You're introduced to a new main character and several new mechanics, the most important of which is factional gameplay, which allows you to ally with an AI-controlled group and assault its enemies. Unfortunately, new features come with a price: new bugs, erratic difficulty, and other annoyances further disrupt the immersion. While Clear Sky is a good game, it's disappointing that developer GSC Game World failed to address the problems with Shadow of Chernobyl that were left to the modding community to clean up.

You'll join your comrades and capture enemy camps in Clear Sky's all-new faction battles.

If you've already visited The Zone and got lost in its nightmarish world and deliberate pacing, you'll find the landscape is still bleak and uninviting. You play as the silent loner Scar, a survivor of a strong emission originating from the nuclear plant at the center of the zone. His rescuers are the Clear Sky faction, a group of scientists investigating the reasons behind the emissions. And of course, their goal becomes yours as well. During your measured journey through The Zone, you'll visit other factions' headquarters as well, where you'll be asked to assault enemy bases, join their brotherhood, and perform tasks in exchange for information.

With so many bases scattered about, you'll soon discover that compared to its predecessor, The Zone is practically teeming with human life--though you shouldn't take this to mean that it's suddenly a carnival of cheery faces. This meatier population is borne out of necessity: Clear Sky's major new addition is that of factional warfare, in which the various packs of mercenaries (or in this case, stalkers) fight each other for turf control. As a result, you can meet the faction leader and join the team, assuming you've proven yourself worthy. This in turn means better merchant prices and other perks, as well as easier (or harder) passage to certain areas. Thus ensues a Battlefield style tug of war, in which factions fight over controls points in an attempt to take over the other's base.

Joining your teammates in these battles, like almost any combat situation in Clear Sky, is a nail-biting excursion into the unknown. Impressive enemy AI is one of the biggest reasons for this. Enemy stalkers make excellent use of cover, crouch and move away when they reload, flank you whenever possible, and generally react in plausible ways. They're tough cookies, so even at standard difficulty, you can't play as you would a standard first-person shooter. The only successful approach is to act and react as you would in real life: with caution and perseverance. Because of the slow flow of cash, you'll often feel spectacularly underpowered, but while these fights are often tense and difficult, clearing out a base with nothing but a pea-shooting pistol and an underpowered hunting rifle feels like a major accomplishment. Just be prepared to save. Often.

Enemies like these invite some of the tensest moments.

Over time, you'll find and purchase new weapons and armor, and can upgrade them with a visit to the right technicians at various bases. Unfortunately, enemy stalkers have a few tricks up their sleeves that border on the magical. Grenades are a nice addition to the series in theory, but foes have an uncanny knack for throwing them directly at your feet from huge distances. The implementation seems a bit off here; grenades explode very quickly once they land, yet S.T.A.L.K.E.R. has never been about quick movement (a stamina bar limits how much you can sprint and jump), so more often than not, multiple grenades usually means game over. The AI's uncanny ability to land shots, particularly at nighttime, also borders on the absurd.

These issues are most prevalent in two frustrating sequences that bottleneck progress in the first half of the game. In one, you emerge into a new area only to find yourself under attack from a nearby military installation. You're meant to run, but the eagle-eyed soldiers riddle you with bullets time and time again as you die and reload, wondering what the secret for escape is. In another, you must take potshots at enemies from behind a bus while dodging multiple grenades at once. Furthermore, this battle takes place a few feet from an exit point to another region; should you accidentally trigger it, your location will be reset to a few feet away, potentially on top of a grenade. Usually, Clear Sky is a tough challenge that makes you feel powerful when you succeed at your task. Sequences like these do the unforgivable: They yank you from the powerfully immersive world at the heart of the experience.