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.

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.

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.

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.

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.

[4] Reviewers cited technical problems with Killzone, including inconsistent AI,[15] occasional bugs, frame-rate issues, distracting graphical glitches,[15] repetition of the same voices,[12] short draw distance, and an awkward control system.

[15] Reviewers such as GameSpy[13] claimed that Killzone partly suffered due to the incredible publicity it received before release, raising expectations only for them to go unfulfilled.

Despite this, Killzone was admired for its sound effects, its soundtrack, and its presentation of a gritty war zone;[15] it was also credited for its unique hard sci-fi art design.