Setting Killzone: Mercenary takes place on the planets Vekta and Helghan, locked in an
interstellar war. The game is set in between
Killzone 2 and
Killzone 3 and revisits many of the key events of
Killzone,
Killzone: Liberation, and
Killzone 2 from the perspective of Arran Danner, a
mercenary hired to execute operations for the ISA. He is supplied by mysterious weapons dealer Blackjack, and is aided at times by his boss Anders Benoit.
Story The game starts with the Helghast invasion of Vekta. Mercenaries Arran Danner and his partner Damian Ivanov are tasked by their boss – Anders Benoit to arrive in the Vektan city of Diortem on a mission to rescue ISA Admiral Alex Grey from Helghast forces using location information gained by the Vektan Ambassador – Sepp Harkin. Danner and Ivanov fight their way through the Vektan Halls of Justice where she is being held. They witness Helghast Colonel Kratek attempting to execute her, but manage to extract her safely. The two then make their way to a downed Helghan cruiser which is attempting to escape with stolen ISA weapons technology. The cruiser also has the transmission codes for all of the Helghast forces so obtaining the codes is vital to the ISA. Danner hacks into the cruisers computers and obtains the codes, however whilst sabotaging the cruiser power supply Ivanov is caught in a booby trap and sacrifices himself to destroy the ship. Despite the casualty the mission is deemed a success. Two years later the
theater of war shifts to Helghan, the Helghast home planet. Danner disables part of the Arc Cannon system in order to open a window for invading ISA forces as because of them the invasion has stalled. He then attempts to rescue the Vektan Ambassador and his family (as the leaking of Admiral Grey's location has been discovered by the Helghast) from the Vektan Embassy in Pyrrhus City, however the Ambassador and his Helghast wife are killed in the crossfire except for their son, Justus, meanwhile a Helghast scientist and defector named Savic escapes the embassy in the chaos. With the help of the Ambassador's bodyguard 'Boris', Danner and Justus make it to the boats beneath the Embassy and escape. Rather than make their way directly back to ISA Headquarters, Danner is ordered by Benoit to divert and take Justus to a Helghast 'smoker tower' where Savic has escaped to as he wishes to defect and has information which is vital the ISA. They need to extract Savic before the Helghast find him, hence the urgency. Danner and Justus make their way through the Helghast infested cliff sides where they eventually find Savic in a bar. Savic reveals that he had created a weaponized virus capable of wiping out the entire population of either planet due to genetic targeting of either race's DNA, and fled due to his conscience. He rendered it inert before defecting, with the key to reactivating it lying inside Justus' bloodstream. Savic had hoped that if Kratek had the inert base of the virus and the ISA had the trigger (in Justus's bloodstream) then neither side would be able to use the virus. Danner engages the Helghast forces which provides enough of a distraction that Savic and Justus safely make their way to the extraction point where Admiral Grey personally meets them. Afterwards Danner's next mission is to a nearby Petrusite reactor complex to cut the power to the remaining air defenses. However, as he destroys the final reactor, Benoit cuts him off and leaves him for dead as Admiral Grey knows that Danner is the only other person who knows about the controversial virus. Kratek overhears this and retrieves Danner, revealing Grey was planning to use the virus to finally wipe out the entire Helghast population. Kratek has Danner return to the Embassy (which Grey has made her HQ) with a Helghast commando team to obtain the codes to the virus vault from Savic, who was being interrogated for the same codes. Savic gives the codes to Danner after he makes Danner switch off his comms so that Kratek cannot hear the codes. Danner can then choose to leave Savic or execute him. Danner observes Justus being taken to a secret Helghast mobile research facility by Admiral Grey and Benoit so they can activate the virus. After initially being pinned down by some ISA
Exoskeletons, Danner is saved by some Helghast tanks. Danner is then taken by
dropship to the Helghast mobile research facility where the virus is held. Following a crash landing, Danner makes his way through the labs and finally obtains a sample of the virus for Kratek. After a firefight Danner escapes, killing Grey in the process, and rescuing Justus. However, the explosives planted by Danner used to destroy the virus activated the explosives planted by the ISA which causes the facility to veer off course. Blackjack, Danner's arms dealer, reveals that Benoit intends to take over the
private military company that hired Danner and monopolize the industry. He also informs Danner that Kratek intends to betray him as well (as he only kept Danner alive for knowing the codes to the vault) and seize the completed virus for himself. Danner and Justus fight their way to the flight deck to steal Kratek's dropship which is the only way off the cruiser. Upon reaching the flight deck, Benoit kills Kratek with the intention of seizing the virus and selling it on the black market. Benoit offers Danner a choice: His life or Justus'. Danner only wants the virus destroyed. As the Helghan capital of Pyrrhus is destroyed in a nuclear blast, Danner kills Benoit and several of his mercenaries after a protracted fight. Blackjack extracts Danner and Justus using Kratek's dropship from where Danner destroys the virus to maintain the status quo. Blackjack also erases Danner and Justus' identities as they flee to safety. In the closing sequence, he remarks that he profited off the sale of anti-radiation medication in the wake of the nuclear detonation, and they all stand to continue
getting rich from the continuing war. Danner and Justus then go 'off grid'. ==Development and release==