Killzone is a sci-fi-themed first-person shooter. The game's stand-out feature is the ability to control any of the four different protagonists, each with his or her own strengths and weaknesses. Up to sixteen players could compete in the online multiplayer portion, which includes eight maps and six modes: Deathmatch, Team Deathmatch, Assault, Defend and Destroy, Supply Drop, and Domination. The online servers of the game have now been closed down.

During the era of space exploration, a faction of humans colonized the planet Helghan. In order to survive in the harsh environment, these people mutated to the point of no longer remaining human. They developed a passionate hatred for mankind, and eventually formed the Helghast Empire, attacking the planet Vekta controlled by the Interplanetary Strategic Alliance (ISA). As Helghast soldiers invade the planet's perimeter, the ISA forces are trying to hold the ground and find a way to stop them.