After fighting his way through the Helghast occupying the exterior of Central Command, Jan runs into Luger, a former comrade who is now working with an elite ISA division known as the Shadow Marshals. As Hakha is the only individual with knowledge of how the invasion bypassed SD, he assigns Jan to locate him. He also reveals that Colonel Gregor Hakha, a half-Helghan intelligence officer, had, on his orders, infiltrated the inner circle of Third Army commander General Joseph Lente, only to vanish while traveling to an extraction point in Vekta's slums. Vaughton discloses that the ISA has requested assistance from the UCN and are working to restore the defense network. While taking part in an offensive to slow the Helghast assault, Captain Jan Templar, a veteran ISA officer, is summoned to a meeting by his mentor and close friend, General Bradley Vaughton. With the element of surprise on their side, the Helghast quickly overwhelm the unprepared ISA ground forces and capture several strategic locations, including ISA Central Command. Alerted to the attack, the ISA attempt to prevent it with their SD (Solar Defense) network, but are unable to activate it in time to stop the invaders. Scolar Visari, emperor of Helghan, sends the Helghast Third Army to launch a secret invasion of Vekta. Violently xenophobic and convinced of their superiority, the Helghan consider humans to be beneath them, and dream of one day reconquering Vekta and expanding their empire to Earth and the neighboring star systems. Eventually, the population, now known as the Helghast, mutated into pale-skinned hairless humanoids with increased strength, stamina, and intelligence. The harsh environment and atmosphere killed many Helghans, forcing the survivors to use respirators and air tanks just to breathe. In response, the exiled colonists established their own civilization on Helghan, built on the principles of militarism and authoritarianism. The Helghan Corporation sought to buy ownership of Vekta as well, but when the UCN imposed sanctions against its unfair business practices, a war broke out (known as the First Extrasolar War), which led to the ISA, the military arm of the UCN, driving the company out of Vekta. Partnering with private firms, the UCN moved to establish human colonies in Alpha Centauri, a system occupied by two planets: Vekta, a rich Earth-like world (named after the CEO of the mining conglomerate Helghan, Philip Vekta), and Helghan, a barren wasteland named after the same company. After nuclear war rendered much of the Earth uninhabitable in 2055, world governments formed an international order known as the United Colonial Nations. Killzone takes place in a fictional world set in the year 2357.

Its direct sequel, Killzone 2, was critically acclaimed. Despite mixed reactions, Killzone spawned numerous sequels. Upon release, however, the game was met with divided responses, with most critics praising Killzone's visuals, art design, soundtrack and atmosphere, but agreed that the final product was disappointing, with criticism also aimed at the game's narrative, characters and gameplay. Prior to its release Killzone was heavily anticipated with several publications considering it to be Sony's ' Halo killer' title.

The game is played from a first-person view and follows Jan Templar, a high-ranking officer within the Interplanetary Strategic Alliance, as he battles invading Helghast forces on his homeworld of Vekta. Killzone takes place in the middle of the 24th century and chronicles the war between two human factions the Vektans, and the Helghast. The game was remastered in HD by Supermassive Games and re-released within the Killzone Trilogy for PlayStation 3 as well as a standalone PSN title in 2012. Killzone is a first-person shooter, developed by Netherlands-based Guerrilla Games and released on Novemin North America.

Killzone 1