The series is set in the 24th century, showing the galactic war between the Interplanetary Strategic Alliance (ISA) and the Helghan Empire.

Shadow Fall features an online co-op survival mode for up to 4 players; however, it can only be accessed by purchasing the Season Pass.

The game is set in 2357, where the Helghast Empire has recovered from its defeat in the First Extrasolar War and launched a blitzkrieg against the outer Interplanetary Strategic Alliance (ISA) colony planet Vekta.

platforms failed during the initial assault, allowing the Helghast to land swarms of soldiers onto the surface and making it difficult for the outnumbered ISA forces.

Captain Jan Templar, the main protagonist, and his squad are ordered back to the base for reassignment, and are sent to find the ISA operative Hakha and the key in his possession.

Templar meets other characters who assist him such as Shadow Marshal Luger (a female special operations assassin), a heavy weapons specialist Sergeant Rico Velasquez (a Helghast-hating soldier with an itchy trigger finger), and Colonel Hakha, a half-Helghast, half-Human spy.

The main protagonists is 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.

Captain Jan Templar has been promoted to Colonel and is the Fleet Commander of the New Sun, a Cruiser airship set up in the clouds of Helghan over its capital city, Pyrrhus, preparing for a final attack.

A cold, barren, unforgiving world, Helghan provides a ready defense with thick, acidic air, dust clouds, and violent surgical lightning storms.

Helghast forces have mounted a devastating counterattack, prompting the ISA (Interplanetary Strategic Alliance) evacuation of planet Helghan.

The protagonist, Lucas Kellan, is orphaned when he and his father, Michael, are spotted with Thomas Sinclair, a Shadow Marshal, who attempts to lead them out of the side that was being taken by the Helghan forces after the "Terracide" that occurred in Killzone 3.

Echo helps him escape to find out that Jorhan Stahl has a "weapon" designed that would annihilate Vektans and half-breeds, which he considers traitors.

The enemy controls large parts of the planet Vekta, and though the ISA armies are fighting hard, they are losing ground.

Jan Templar returns as the main protagonists on a covert operation to save hostages captured by Metrac, while ISA troops continue the fight for liberty.

In the game players assume the role of a mercenary who fights alongside either the ISA or Helghast forces depending on which team hires them.

PSM stated that Killzone is "Graphically stunning and hits the mark in so many ways, it's baffling; they promised us we'd experience future war, and we have... and come away shaking."

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

[14] Sarcastic Gamer's "How to Kill a Brand" (a parody of How to Save a Life) criticized Killzone, saying it didn't deserve a sequel.

Killzone: Liberation received positive reviews, earning IGN's award for Best PSP Offline Multiplayer Game of 2006.

The review stated "on a visual level, as a 3D breakthrough, this feels like a watershed moment in a revolution affecting all of gaming".

[17] Killzone: Mercenary received positive reviews upon release, with critics praising the quality of the title compared to previous shooters on the Vita.