This page contains story elements that may spoil the experience for you.
Watch out while reading this, or don't at all, until you know what's coming.
Starting just after the ending of the original Killzone, and revisiting many of the events from the Killzone trilogy, the game puts players in the rugged combat boots of a mercenary named Arran Danner. Danner, a former UCA soldier, has no qualms about taking on paid contracts from the ISA, as well as the Helghast.
The game starts off during the Helghast invasion on Diortem. Led by Helghast Colonel Vyktor Kratek, the Helghast forces have breached into the Vektan Halls of Justice, in an effort to capture ISA Admiral Alex Grey. It isn't long until the overwhelmed ISA request the aid of the Phantom Talon Corp, an elite group of mercenaries led by Anders Benoit. Shortly after, mercenaries Arran Danner and Damian Ivanov are sent to retrieve the admiral. Danner and Ivanoff make their way to a skyscraper opposite of the building where Grey is being held, and fly down to it in glide suits. They eventually arrive at the Halls of Justice, where Grey is being interrogated. Unfortunately, they are too late. Kratek has already escaped, but not before nearly-fatally shooting Grey. Thankfully, the duo still manage to extract Grey, and the mission is a success.
Fortunately, there is still some work to do in Diortem. After being shot down by ISA forces, a crippled Helghast cruiser lays stranded in the center of the city. On board, a collection of stolen weaponry is being hauled for the trip back to Helghan. Danner and Ivanov are once again requested for the job. After making their way through the cruiser, Danner and Ivanov eventually reach the server room. Here, the two manage to extract transmission codes, which were being used to contact the Helghast fleet. Unfortunately, Ivanov triggers an alarm, causing a quarantine. Ivanov is left for dead, while Danner manages to escape in time. Despite the casualty, the mission is still a success.
Two years later, and the ISA have brought the fight to Helghan. Unfortunately, the invasion is stalled by a collection of Helghast arc cannons. Danner is the sent in, this time with a new mercenary named Carrell. Unfortunately, Carrel is shot down while gliding past the cannons. Nevertheless, Danner still manages to make it to the platform. After infiltrating the station, Danner takes out the arc cannons. After a short firefight, Danner is once again extracted by Benoit, via an intruder.
Soon after, Danner is sent to a Helghats embassy, where ISA Ambassador Sepp Harkin is trapped from Helghast forces. Unfortunately, Danner arrives too late, and Sepp and his wife Valeria are dead. Thankfully, their half-breed son Justus survives. With the aid of the family bodyguard Borris, Danner manages to get Justus out alive. The two then escape the embassy via Helghast boat.
Following the unfortunate events at the embassy, Danner is forced to change course for a nearby fishing village. Here, Danner is ordered to locate and extract Helghast scientist Mandor Savic, who evacuated the embassy before Danner arrived. After slowly making his way through the Helghast-infested cliff sides, Danner arrives at a bar, where Savic is hiding. After escorting Savic and Justus out the bar, Danner decides to stay and fend off Helghast forces. After holding the fort at the bar, Danner makes his escape. Once outside, he is extracted by Benoit.
Shortly after, Danner is once again requested by the ISA. This time, he is sent to a Helghast refinery. Here, he is tasked with stopping the manufacturing of Helghast weaponry. After planting charges, however, he is left for dead by Benoit. With the refinery imploding on itself, Danner sprints for the exit. Thankfully, Kratek proposes Danner with a deal. In exchange for his life, Danner is forced to work alongside the Helghast. Danner accepts the contract, and escapes the refinery.
Now with the Helghast, Danner is taken back to the embassy, where the ISA and Helghast are battling it out for Savic, who is now being interrogated by the ISA. After infiltrating the embassy, Danner seeks out Savic. Once the two reunite, Savic reveals a shocking truth: Justus is the key to a deadly virus. Hidden in the boy's bloodstream is a code, which, when activated, can trigger the virus. Once Danner retrieves the code from Savic, he begins his escape. After being pinned down by some Exoskeletons, Danner is saved by some Helghast tanks.
After retrieving the codes from Savic, Danner is taken to a secret ISA cruiser, where the virus is being stored. Following a crash landing, Danner shoots it out, with the aid of some Helghast. After making his way through several labs, Danner finally reaches the virus. Unfortunately, Grey is right on his trail. With the virus in hand, danner kills Grey, and begins to make his way out of the cruiser, before it's inevitable demise.
After reuniting with Justus, the two begin to make their way out of the cruiser. After facing off against several waves of Helghast, Danner eventually reaches an elevator, leading up to the top of the cruiser. Unfortunately, Benoit and Kratek are one step ahead of Danner. Held at gunpoint, Danner is forced to make a difficult decision: His life, or Justus's. Before Danner can choose, however, Benoit kills Kratek. After some hard thought, Benoit has decided to sell the boy to whoever bids the highest, rather than to any faction in particular. Danner, on the other hand, only wants the virus destroyed. The two battle it out, until Danner eventually kills Benoit. Danner and Justus are then extracted by Blackjack, Danner's friend and supplier. Moments later, the cruiser crashes, alongside the virus.
Now fugitives, Danner and Justus are kept under the radar by Blackjack... for a price.
Mercenary is made up of three distinct modes: campaign, contracts, and multiplayer. Mercenary's campaign is made up of nine missions, all of which can be played on various difficulties. Once the player completes a mission, they are given access to three contracts. These contracts require players to replay missions, while completing demolition, covert, or precision-related objectives. Unlike previous titles in the Killzone series, players now earn in-game cash for completing various tasks (e.g. killing hostiles, picking up ammo, using explosives). In terms of multiplayer, players are given a total of six medium-sized maps. Mercenary's multiplayer modes consist of Mercenary Warfare (free for all), Guerilla Warfare (team deathmatch), and Warzone (objective based deathmatch). Also new to the series is the valor card system. In multiplayer, deceased hostiles and allies will drop a valor card. Once the player has picked them up, the cards will be instantly added to the players "deck". Once a player has completed a deck, they are rewarded with large cash rewards. Mercenary also takes advantage of the PlayStation Vita's features. Brutal Melee can now be executed via the Vita's touchscreen. Swiping in a specific direction within the time limit will result in a successful kill. Interrogations are also carried out via the touchscreen. Additionally, sprinting can also be accessed by double tapping the Vita's rear touch pad.
At Gamescom 2012, it was revealed that the title is Killzone: Mercenary. The game utilizes a modified version of the Killzone 3 rendering engine, allowing volumetric lighting and smoke, high-res environment textures, gleaming metal, and realistic-looking shadows. Unlike the previous Killzone installments, Mercenary was not developed by Guerrilla Games, but instead, their sister studio, Guerrilla Cambridge (formerly known as SCE Cambridge Studio). Because of the refined damage model, the weapons apparently "feel slightly more powerful" than those in previous installments.
In the weeks prior to Mercenary's release, Guerrila Cambridge hosted a private beta for select Vita owners to participate in. The beta featured practically all the contents released in the final product. The beta was meant to promote Mercenary, as well as to inform Cambridge on any requests related to Mercenary's multiplayer. About a week before launching, Guerilla gave PS Plus members free access to the beta, before making it entirely public.
Killzone: Mercenary was received well by critics. IGN's Colin Moriarty gave it an 8/10, calling it the greatest handheld shooter on the market. Destructoid's Jim Sterling also gave it an 8/10, calling it ambitious. It currently holds an average score of 78/100 on Metacritic. TheSixthAxis stated "there’s no other experience like this on a device as small as this". It received the 1st Place 2013 PlayStation Vita Game of the Year Award. 
In terms of sales, Mercenary had a strong launch. It has so far raked in over $350,000. It's currently the 15th best selling Vita game. 
- For the first time in a Killzone campaign, players will be fighting alongside both Helghast and ISA forces.
- It is the first first-person shooter Killzone game on a handheld.
- It is the only game in the Killzone series to feature a silent protagonist.