S.T.A.L.K.E.R.: Clear Sky, is the stand-alone prequel for S.T.A.L.K.E.R.: Shadow of Chernobyl, a first-person shooter computer game by Ukrainian developer GSC Game World. The game consists of a roughly 50/50 mix of new areas and old, remodeled areas from the previous game. The X-ray graphics engine has been updated to version 1.5 and includes DirectX 10 support. Additionally, the AI received an overhaul to accommodate the new faction wars feature.
Clear Sky is a prequel to the events in Shadow of Chernobyl.
The player assumes the role of Scar, a veteran Stalker mercenary who was injured by an energy emission while guiding a group of scientists through the Zone. Scar is rescued by Clear Sky, a secret, independent Zone faction dedicated to researching and understanding the Zone in order for humanity to better understand what kind of phenomena it is facing. After attempting to help a patrol at an outpost, a second emission occurs, where Scar survives. The leader of Clear Sky, Lebedev, informs Scar that the Zone is being disrupted by a dramatic increase in energy emissions, emanating from the Chernobyl power plant in the center of the Zone. Besides endangering all the Stalkers in the Zone, the emissions are also interacting with Scar's body in a unique way that will eventually kill him if they continue. Lebedev theorizes the energy emissions are a sort of "immune response" by the Zone itself in response to human activity. If it continues, the zone could become highly unstable and run out of control.
Scar is tasked with finding information about any activity regarding Stalkers trying to reach the center of the zone. The first clue is that a suspicious group was found talking to a trader named Sidorovich in the Cordon, the southern boundry of the zone. After helping Clear Sky forces regain control of the swamps, Scar makes his own way to the Cordon, where he discovers Loners and Military are squabbling. The result makes a case of loot that Sidorovich wants go missing. If Scar finds the case, Sidorovich will be able to shed some informaton. To do this, Scar works with Loners by removing Military forces attempting to rescue the captured commander, Kheletsky. The location is revealed and Scar finds the case, returning it to Sidorovich.
Sidorovich explains that the group were looking for some exotic parts. He directed them to the Garbage, where piles of old relics and unwanted objects are left lying around for looting. One of the member's names was Fang. Scar heads to the Garbage to try and find Fang, but only stumbles along a PDA that belongs to the group. He finds the group went into the Dark Valley. Scar heads to the Dark Valley to find the Freedom faction, trying to control the area from mysterious attacks and losses of numbers of comrades. Not only are bandits found being used as a smokescreen, but mercenaries sent upon Freedom by a renegade commander are behind the job. Scar helps Freedom deal with the mercenaries and the commander in order to retrieve more information. The group returned to the Garbage shortly after visiting the Dark Valley from searching for more materials. Scar heads back and heads for the Flea Market, owned by Loners.
When he reaches a basement, he is ambushed by bandits and has his equipment taken, but is unharmed. Lebedev wakes Scar and tells him Fang's PDA is nearby. Upon discovering it, the factions identity is revealed. Strelok Faction, a small group of elite Stalkers, are attempting to penetrate to the center of the Zone. According to the log, they have done it once before and are now going to do it again by force. The zone was trying to consume them in order to stop their progress, but its not known how they got past the Brain Scorcher. Their intentions seem to indicate they are going for the Wish Granter. In order to bring an end to the emissions and save his own life, Scar is tasked to journey to the center of the Zone and _Shadow_of_Chernobyl#Plot. Before that, Scar will need to recover his equipment from the bandits who stole it.
He travels to Agroprom, which is in the hands of Duty. The zone is being overwhelmed by very dangerous mutants and the commander is hard pressed trying to remove them. When it is found that Strelok went to the underground, Scar volunteers to head into there to rout out the mutants. He discovers another PDA which confirms the groups intentions of reaching the Wish Granter and the power plant. After escaping the underground, Scar must head north, knowing there will be many places to get through in order to reach the NPP.
His first stop is Yantar. The factory near the science bunker is being swarming with zombies, ordinary people whose brains have melted due to a powerful psi field there. Loners are protecting Professor Sakharov, who is operating in the area. The Professor tells Scar the the faction did pass through here, notably the leader, Strelok, who volunteered to test a prototype device. The device would allow the user to completely resist and block psi emission effects, like the Brain Scorcher. To better understand this, Scar finds notes of a group of dead stalkers near the plant that discusses the effects of the psi-field and explains why the local emissions are happening. In order to proceed, the faulty cooling system of the psi-emitter beneath the factory must be repaired so that the emissions cease. He teams up with a Loner named Lefty and his crew who restart the coolant pumps inside the factory. Without the emissions, there should be no more zombie problems and Scar can proceed to the Red Forest.
Strelok is directly in his sights, threatening Scar with death if he continues his pursuit. After chasing him to a motorized bridge, an ambush springs and Scar must fend it off. Strelok escapes unscathed and alone, blowing up a tunnel. He assures that Scar will not follow he'll reach the center with no one stopping him. Lebedev makes contact and tries to think of a way over the other side of the bridge, which is raised, over highly radioactive water and controlled by bandits. The bridge is the only other way to the center, through a town called Limansk. Scar is directed to an old Forester in the Red forest, protected by Duty, who may know a way. Forester tells Scar he knows of a way. A group of Stalkers guided by a mercenary called Leshy was attempting to reach the other side but ended up in a spatial anomaly, effectively trapped somewhere nearby Limansk. After recovering a unique artifact for Forester which enables him to navigate the mercs out of the anomaly, a plan is made to free Leshy and his crew.
Scar is directed to a number of Military warehouses, where he finds some mercenary friends of Leshy. They know of a military radio transmitter which is powerful enough to transmit the needed coordinates to Leshy's team. The leader pairs Scar up with a small band of Freedom soldiers, who are making a raid on a small Military Base. The forces there are just a small portion of the only Military left. After clearing the base, Scar powers up the transmitter and Leshy is freed. They are now on the other side of the bridge, waiting to carry out their plans. Lebedev and a large contingent of Clear Sky soldiers head to the bridge, awaiting Scar's arrival so they can make a push. When he does get there, he assists the Clear Sky forces providing cover for Leshy's team while they lower the bridge. Afterwards, Scar moves out with the advance team into Limansk.
The pressure is on. Limansk has turned into a street war between bandits, remaining Military forces and, if Scar did not encounter them in the Military Warehouses, the mysterious and powerful Monolith faction. Clear Sky helps Scar push through the war-torn town so he can get one step closer to the center. After making it out, the only thing that's left is an abandoned hospital. Another advance team is pinned down by Monolith, who are swarming in the hospital. Scar and Clear Sky forces assist one another to make a push through the hospital so they can get to the NPP, on the other side. But on the way out, a helicopter cuts them off. After Scar defeats the helicopter, the team proceeds.
The final battle is at hand. Strelok is within the vicinity, defending himself from both Monolith and Clear Sky. Lebedev thanks Scar for helping the reach the center and now needs his help to finish this. Given a special energy rifle, Scar must use it to disrupt the psy-blocker device on Strelok. Without it, Strelok will not last much longer and the psi emissions in the zone should subside. After several direct bursts of the energy rifle, Strelok is defeated and readings are taken. But suddenly, an unexpected and massive emission occurs. It is said the area should have stabilised. Everyone is hit and begins succumbing to the effects of the emission. From indications at the end, Lebedev, Clear Sky and Scar either die, are brainwashed to become part of the Monolith forces, or, like Strelok, become unwitting agents of the C-Consciousness.
The most significant gameplay addition since Shadow of Chernobyl is the faction wars system. Different factions will struggle for territory, attacking to gain territory and then defending to keep it, while others then try to retake it. The player will be able to join and help factions in their battles. The stronger a faction becomes, the better equipment the traders can provide and their soldiers can use. The player character is a mercenary and may do missions for any faction, or remain completely neutral, without consequence. Each of the main factions provide services, most importantly access to a trader and an engineer. Some factions, such as the military, cannot be joined.
While Scar is always aligned with Clear Sky, and his ultimate goal is to defeat the Strelok Faction, he can fight against or ally with the four other factions in the Zone (Loners, Duty, Freedom and Bandits). The Swamp-dwelling Renegade or the Military factions cannot be joined.
Other gameplay advancements since the first game include a deepened weapon customization system with the ability to repair damaged gear. Anomalies are harder to notice and now contain the artifacts in the game, which require a detector to locate. NPCs are given the ability to use hand grenades, take cover dynamically and use "blind-fire" techniques. Light machine guns have been introduced. There are NPC guides in the zone that will provide fast-travel for a fee.
Since 1.5.03 version game supports MSAA for DirectX 10.
On September 5th 2008 the game was officially released in the rest of the world, except in North America and on various online distributors. Reports of poor performance are still prevalent around the official forum and some of the fansites. Since then the game has been patched several times and the latest is now currently 1.5.05, fixing most of the gamebreaking bugs that were present upon release. However some players are continuing to report severe bugs with the game, including severe graphical issues that were not present in 1.5.04.
The Russian limited edition is presented in a larger box and contains the game disc, a bonus disc, two kerchiefs, an A2-sized map of the Zone, several patches with logos of the game's factions, a dog tag, a custom lighter and a little white ball called "The Clear Sky Artifact".
The second limited edition, released in the rest of the world is presented in a metal box and also contains a bonus disc filled with extras (such as bonus artwork, screen savers and making of videos) and the A2-sized map of the Zone.
Polish limited edition, presented in medkit-like bag with game's logo contains game disc, soundtrack, patch with logo of one the game's factions, stickers with logos, small map of the Zone, t-shirt with game's logo on chest and inscription "Сделано в Чернобылье" ("Made in Chernobyl") on back.