Set in the near future during a period of planetary colonization, Killzone drops you in the middle of a war between the ISA Command and a militaristic separatist faction, the Helghast. As one of four ISA members, your initial objective is to escort a high-ranking official to an evacuation point. You can experience several gameplay modes, including single-player, multiplayer (team-based and competitive), and online play for up to 12 players. Players can equip themselves with an armory of more than 27 different weapons based on modern technology.

The game is set in an era of space colonization where the Helghan Empire has recovered from its defeat in the First Extrasolar War and launched a blitzkrieg against the outer ISA colony planet Vekta. Vekta's orbital Strategic Defense (SD) platforms failed during the initial assault, allowing the Helghast to land swarms of soldiers onto the surface. This makes it all the more difficult for the outnumbered ISA forces.

In the game, the Helghast are a faction of human descendants who colonized the planet Helghan many generations ago. The planet's harsh environment forced the Helghast to adapt and mutate so much that they can no longer be considered human. They are stronger, faster and more resilient than their cousins, and possess a burning hatred for humanity. Except for a small number of half-breed Helghast, the few social elite born commanders and some trained troopers, they all require a gas mask and air processing tank that creates air similar to the planet Helghan's atmosphere for long term period away from the planet. Unprepared, outnumbered and outgunned, Vekta's only hope rests with an elite squad of four soldiers who will turn the tide of the war.

As the climactic war razes across Vekta, Templar allies himself with troops on the outskirts of Vekta City, desperately repelling endless waves of besieging assaults from the imperious Helghast forces, but are eventually driven into a defensive retreat.