Like the previous installment, S.T.A.L.K.E.R.: Clear Sky is set within the Zone of Alienation in Chernobyl, Ukraine in 2011, five years after the second Chernobyl disaster. The player controls a mercenary known as Scar, employed by the Clear Sky faction to go after a group of stalkers attempting to breach in the center of the Zone.
The gameplay of S.T.A.L.K.E.R.: Clear Sky is mostly reminiscent of that of Shadow of Chernobyl, but benefits from a new set of features with the new engine. The game area is composed of many remodeled areas featured in Shadow of Chernobyl, including the Chernobyl Nuclear Power Plant. New areas include the Great Swamp, the ghost town of Limansk-13, and the Limansk Hospital, though locations such as Rostok and Pripyat do not make a re-appearance.
The survival aspect of the game includes salvaging supplies, such as ammunition, weapons, food, and medical supplies. These can be looted from the bodies of dead stalkers, found in stashes, or bought from traders. Money can be acquired by completing optional missions and selling gear to traders. A new addition to the game is a repairing and upgrading system, allowing the player to repair weapons and armors, and to upgrade their statistics through technicians. The upgrade tree follows a precise path that involves mutually exclusive trees of modifications, typically involving one dedicated to improving accuracy and the other to improving damage statistics. Picking one tree will lock the player out of selecting the other one.
Lebedev is amazed by Scar's survival and theorizes that the Zone is being disrupted by a dramatic increase in energy emissions emanating from the Chernobyl Nuclear Power Plant in the center of the Zone. Human activity is believed to be the cause. In other words, the emissions are part of an \"immune response\" by the Zone itself. Someone has made it to the center, and the Zone itself is trying to consume that person in order to preserve its secrets. Besides endangering all the Stalkers in the Zone, the emissions are also interacting with Scar's body and immune system in a unique way that will eventually kill him if they continue. Furthermore, if the emissions continue, the Zone could eventually become highly unstable and run out of control.
When he reaches the basement of the flea market, he is ambushed and rendered unconscious by Bandits. The Bandits laid their trap for Fang, but catch Scar instead. They take his equipment and run, leaving Scar unharmed. Lebedev wakes Scar and tells him Fang's PDA is nearby. Upon discovering it, the faction's identity is revealed. The Strelok Faction, a small group of elite Stalkers, are attempting to penetrate the center of the Zone. According to the log, they did it once before and are now going to do it again by force. Lebedev confirms that they are the ones the Zone is trying to destroy, but it's not clear how they got past the Brain Scorcher or why exactly they are doing this. Upon finding that they went towards the Agroprom Research Institute, Scar must pursue the team by any means possible. But before that can happen, Scar will need to recover his equipment from the Bandits who stole it.
Scar travels to the Agroprom Institute, which is in the hands of Duty, the rival of Freedom. The Zone is being overwhelmed by very dangerous mutants, and the commander of Duty is hard-pressed, trying to remove them. The Duty commander, General Krylov, tells Scar that Strelok went to the Agroprom Underground. However, the underground is a dangerous area, full of mutants who attempt daily to breach the Agroprom, killing Duty stalkers. Scar volunteers to head in there and rout out the mutants, in exchange for further information. Battling through mutants to get inside, Scar then floods the underground to kill the mutants in the area. After further exploration, he then discovers that Strelok had a hideout in the underground, in which he finds a PDA that confirms the group's intentions of reaching the Wish Granter and the power plant. Their intentions clear, Lebedev gives Scar the undeniably difficult task of killing Strelok before he reaches the center. While Scar pursues him, Clear Sky will ambush and stop the others. After escaping the underground, Scar heads north, knowing it will be a long road to travel in order to reach the CNPP (Chernobyl Nuclear Power Plant).
Scar is directed back to the Military warehouses, where he reunites with the Mercenary friends of Leshiy. They know of a military radio transmitter that is powerful enough to transmit the needed coordinates to Leshiy's team. Unfortunately, it is guarded at a base by what is believed to be the last of the Military. The leader pairs Scar up with a small band of Freedom soldiers commanded by Kostyan, who are making a raid at the same place in order to secure the area for Freedom. After fighting heavy resistance at the base, Scar powers up the transmitter and waits as Leshiy and his team free themselves. They then inform Scar that they are now on the other side of the bridge. Lebedev acts immediately on this development. Convincing Leshiy and his team to aid Clear Sky in reaching the other side, a large contingent of Clear Sky soldiers heads to the bridge, awaiting Scar's arrival so they can make a push. When he does get there, he assists Lebedev and the Clear Sky forces to provide cover from Bandits snipers for Leshiy's team, while they lower the drawbridge. Afterward, there are many thanks all around and the path is clear. Scar moves out with the first advance team heading into Limansk.
The pressure is on at this stage. Limansk has been turned into a street war between Bandits, remaining Military forces, and the Monolith faction. Clear Sky helps Scar push through the war-torn streets, having to face many problems. From clearing out a machine-gun nest, Scar has to survive multiple Monolith ambushes until he reaches a partially-constructed multi-story building, heavily defended by Monolith forces. After making it out, he helps cut power to electrical fences blocking the way out of Limansk. After he has made it through, the only thing that's left is to take an underground passage located just past an abandoned hospital. Another advance team is pinned down by Monolith, who are swarming in the hospital. Scar and Clear Sky forces work together to push through the hospital to reach the NPP on the other side. There is harsh resistance, but Scar pulls through. But during the process, a Mi-2 controlled by the Military appears. It opens fire on everyone in the Hospital, including Scar. Scar defeats the helicopter alone while the Clear Sky team catches up. As they reach the tunnels, the team stays behind, giving Scar a window to reach the NPP.
Scar, the game's protagonist, was known instead as Messer, a guide leading International Scientific Group stalkers (United Nations, basically) on a mission deep into the Zone, to the Generators, a place where time and space were not bound by laws of physics.
Suddenly, a blowout took place and Messer reappears on the outskirts of the Zone, in the Marsh, and is rescued by Clear Sky and recruited to track down Strelok before his incursions into the heart of the Zone can destroy it. However, it is not the real stalker - he has been copied by the noosphere, which gained consciousness in the process. The game progression also differed slightly - after uniting the warring factions, Messer was supposed to find a key that would allow him to reach the Cartographer, a legendary stalker that knew how to get everywhere in the Zone - who would become Forester in the final release, living in a small Orthodox chapel. He would tell the hero how to reach Ghost City, through which the NPP can be reached safely and Strelok intercepted before he can trigger another massive blowout.
A group of stalkers, for the first time, reaches the very heart of the Zone-the Chernobyl Nuclear Power Plant-and triggers a cataclysm on the brink of a catastrophe. An immense blast of anomalous energy transforms the Zone: the once reliable and relatively safe roads are no longer so, the landscape is wiped clean by outbursts of anomalies, and previously unknown areas appear on the Zone map. Stalkers and expeditions perish or end up isolated within the lost territories.
3. NPF( Non-Player Faction ) - AI( Artificial Intelligence ) SystemThe task-based, territories-driven AI System makes the game more real, dynamic, interesting and challenging.Currently the NPF-AI system defines 9 tasks which could be executed in any order. The AI system also supports multi-tasking which means there could be any number of tasks of any kind being executed at the same time.* Protecting Base: when NPF's base is besieged, it would order squads to retreat to defend the base.* Defensive Task: when the important territories of NPF are under attack, it would send squads there to repel the attacks.* Chasing Player: When the player is spotted on the level of his enemy's base. His enemy might send squads after him constantly.* Escorting player: When the player is on the level of his friendly NPF's base, the NPF would send squads to escort him.* Redemption of Allied Factions: When a NPF is defeated( its base is captured by its enemies ), its allied faction, if there's any, would send squads to help it to recapture its base.* Offensive Task: When a NPF acheives supriority, it would send squads to conquer its enemies.* Victory: When a NPF's enemy is defeated, it would order all of its squads to return to its base, thus sparing more territories for other factions.* Increasing Influence: This task is similar to the \"Expansion\" command.* Repelling task: This task is used to repel the NPF's enemies on the NPF's private territories. It would enhance the NPF's defensive ability greatly.The tasks system are very flexible and extensible. New class of task could be defined and added to the AI system very easily.And with the territories-driven AI system, th