Killzone 2 follows the events of Killzone and Killzone: Liberation, and is set entirely on the planet Helghan, the home-world of the Helghast who invaded an Interplanetary Strategic Alliance (ISA) colony. Two years after the Helgast assault on Vehkta, the I.S.A. had launched an assault on the enemy's homeworld of Helghan. The ISA goal is to capture the Helghast leader, Scolar Visari, and bring the Helghast war machine to a halt. Players assume the role of Sergeant Tomas "Sev" Sevchenko, a battle-hardened veteran of the special forces unit the "Legion" assigned to Alpha team, who go on a mission to take out the Helghast threat.
During the invasion of Helghan, Sev's unit is deployed behind enemy lines to assist the main invasion force. Tasked with securing the enemy capital of Pyrhus, Helghan's first true city,the team quickly discovers that the Helghast are a more formidable enemy on their home planet. Not only have they adjusted to Helghan's hostile conditions, they have also harnessed the power of the freakish lightning storms that always occur on Helghan which they can now use against the ISA. A cold, barren, unforgiving world, Helghan provides a ready defense with thick, acidic air, dust clouds, and violent surgical lightning storms. Sev discovers his squad is not just fighting enemy forces—their fiercest opponent may be the planet itself.
In the prologue, the ISA has set up their Cruiser airships, including the New Sun, up in the clouds of Helghan over its capital city, Pyrrhus, preparing for a final attack. Then, the invasion begins. The ISA, including Alpha Team, are deployed in an attempt to capture the city. The invasion is seemingly successful until the Helghast activate their main defense system, a series of arc towers throughout the city, killing many soldiers and severely damaging the first wave of the assault; however, the fight between the ISA and Helghast continues.
Using a piece of an arc tower picked up earlier, scientist Evelyn Batton discovers that the arc towers are composed of an element called Petrusite, and large portions of the mysterious element are located around the outskirts of Tharsis Refinery. The Alpha team is ordered to accompany Evelyn and travel to the outskirts of the city in an attempt to find a way to disable the towers.
During the mission, Seth Meyers and Rico departs from the others in order to activate a communications tower. Before the tower is activated, however, Garza, Natko and Evelyn are captured by Helghast. Eventually, Sev and Rico find their squad: they see Colonel Radec and his guards preparing to execute them. While Sev waits, Rico's impatience causes him to rush in, in an attempt to save the squad. Unfortunately, Garza is mortally wounded and Radec escapes. The only thing they can do is return back to the New Sun. Evelyn heads to the nearest Communications tower to call for extraction. The squad is then attacked by more Helghast troops, but they are able to fend off the enemies. Unfortunately, when ISA support finally lands, Garza collapses and dies in Sev's arms.
Back aboard the New Sun, Sev mourns for Garza; however, it is interrupted by a Helghast attack on the ISA fleet. All ships, including the New Sun, are invaded. In the assault, Colonel Radec reaches the bridge where he kills Evelyn and Fleet Commander, Colonel Jan Templar. Before leaving, Radec steals the nuclear strike codes for the (previously stolen) nuclear weapons. Templar, left to die on the bridge, manages to deactivate the thrusters on the New Sun, sending it falling down to Helghan. The ship as coordinated crashes into Tharsis Refinery, destroying the city's petrusite defense grid.
With the grid down, the ISA advance and attempt to regroup near Visari's palace in Pyrrhus; however, a nuclear bomb is detonated within the city, destroying the city, along with most of the ISA forces. Despite such events, Alpha team and the last remaining ISA attack the palace despite very heavy Helghast resistance.
During the final assault, Sev and Rico severely injure Radec, who chooses to commit suicide to avoid capture.
Finally, Sev and Rico find Scolar Visari who is waiting for them. He tells Sev that the war is far from over, and if they capture or kill him, the Helghast would remember him as a martyr. This would cause the Helghast to fight with more determination in order to destroy the Vektans, and that he is the only one who is keeping the Helghast "under control".
Rico, who decides to avenge Templar along with other ISA members, shoots Visari. As Visari succumbs to his injuries, he mutters his last words: "The madness...begins."
Worn from all the occurrences, Sev leaves the palace and sits on the entrance stairs. There, he sees an overwhelmingly giant Helghast fleet destroying what is left of the ISA fleet and sees, first hand, what Visari envisioned.
- Sergeant First Class Tomas "Sev" Sevchenko is the main character of Killzone 2, and part of the ISA Special Forces Alpha Team. He is a withdrawn and unusually contemplative man. At age 25 he's already been in several successful campaigns, but they've also exacted a toll on him. He's an imperfect soldier in a perfect army.
- Corporal Dante Garza is a member of the ISA Special Forces Alpha Team. Smart-mouthed and young, Garza is a loyal, optimistic and trusty soldier who has earned his elite status. A close companion of Sev, the pair have served together in several campaigns before being picked for Alpha.
- Master Sergeant Rico Velasquez is a veteran of the ISA. He is a human tank who prefers to shoot first and ask questions later, if at all. He's all attitude but despite this he has a good heart. Rico prefers an up-front fight and is a character of extremes; he can get agitated very quickly and doesn't hold back on anything. During the orbital defense platform crisis, Rico was left stranded by a brutal Helghast attack and consequently befriended Templar, Luger and Hakha. Later, he became embedded with Alpha Team to spearhead the invasion on Helghan’s capital, Pyrrhus.
- Corporal Shawn Natko is the demolitions expert for the ISA Special Forces Alpha Team. An experienced soldier of the same generation as Rico, Natko made a steady rise through ISA ranks before joining Alpha Squad.
- Colonel Jan Templar is the commander of the ISA fleet and the cruiser New Sun. Protagonist of Killzone and Killzone: Liberation, he leads the invasion of Helghan from the sky, without actually being in the field.
- Scolar Visari is the leader of Helghast forces and ruler of Helghan planet. He is responsible for the re-militarization of the Helghan troops after their defeat following the First Helghan War. He has succeeded in launching an invasion of one of ISA colonial planets, Vekta, to avenge what he sees as the abandonment and abuse of the Helghast. He is voiced by Brian Cox.
- Colonel Mael Radec is the commander of the Helghast Imperial Guard—Visari's personal guard and defends planet Helghan, he is shown to have a desire for killing and prefers action rather than planning. Determined and ruthless, he is called the hound of Visari. He is voiced by British actor Sean Pertwee.
The game is presented almost entirely from a first person perspective, aside from vehicular combat. Killzone 2 features a "lean and peek" cover system which allows the player to take cover behind an object and then pop out to fire at enemies. The "lean and peek" mechanic stays in first person view at all times. It is also possible for Sev to pilot vehicles at two points in the game: a tank and an exoskeleton. Many classic weapons and vehicles from previous Killzone installments returns, such as the M82-G and the StA-52 LAR. The Sixaxis motion control feature is also utilized in performing certain actions such as turning a valve, arming an explosive charge and sniping. Game director Mathijs de Jonge has confirmed there are currently no plans to have co-op in Killzone 2.
Warzone, the title of the online multiplayer component of Killzone 2 has been developed by Guerrilla Games in conjunction with the game's single-player campaign. The online multiplayer gameplay is class-based, meaning the player can choose a class of character which is specialized for a specific role to better suit the player's needs in battle. There are 7 classes in total, in which the player can mix and match two according to their playing style, and can switch abilities upon death.
There are 15 weapons available to the player, most of which cannot be used until the player reaches a certain rank. Two of these weapons are secondary firearms, and a further two weapons (Boltgun and Flamethrower) are exclusive to the downloadable maps "Suljeva Cliffside" and "Arctower Landing".
Players play as either the ISA or Helghast, with a few gameplay difference, the character model and respawn points, and the "lean and peek" cover system removed. Each class levels up with experience gained from killing or completing mission objectives, with enough experience they unlock new weapons, skills etc. as well as a new class that is unlocked. Each class has two badges, the Primary badge selects the class and their specified skill, the secondary badge has an extra skill for the specified class but can be swapped to create your own custom class.
Warzone plays out through dynamic matches where multiple game types are played in a single round. The game ships with five different game types including Assassination, Search & Retrieve, Search & Destroy, Bodycount and Capture & Hold. Online matches can connect 2 to 32 players, and they can group into six squads of up to four players. If an online match does not have a full 32 players, computer-generated "bots" can be added in to create bigger teams (but only in an unranked match). Killzone 2 also offers a clan system, which allows clans of up to 64 players to compete for "Valor Points", an in-game currency that clans can use to bet on tournaments.
Players can also play offline against AIs with Skirmish mode and unranked online multiplayer but there's no offline multiplayer mode for splitscreen human players.
Eight multiplayer maps are included on disc at release, Guerrilla has stated that further maps will be made available as future downloadable content. A further 6 maps have been added to multiplayer since release via downloadable content purchasable from the PlayStation Store.