Set in 1970 in South America, six years after the events of Snake Eater, the game follows the exploits of Naked Snake after his former unit, FOX, goes renegade. The game also chronicles the eventual founding of FOXHOUND and The Patriots, as well as the inspiration of the military state Outer Heaven.
Unlike the previous titles on PSP, Metal Gear Acid and its sequel, which were turn-based tactical games with stealth elements, MPO retains the action-based gameplay from the console iterations, drawing heavily from Snake Eater and utilizing the camera system from Subsistence.
The main addition to MPO is the Comrade System. Instead of the solo missions that the series is known for, MPO goes for a squad-based approach, with Snake having to recruit allies and form a team of trained specialists. Before each mission, the player must compose a four-man squad. The squad is then sent into battle. Each member of Snake's squad has their own strengths and weaknesses. While some units are best utilized on the battlefield, others may specialize in producing items, healing allies, or providing intel for each of the game's maps.
During gameplay, the player controls only one squad member at a time. The squad members not in use hide themselves inside a cardboard box, and can be swapped into play when the player-controlled character finds a hiding spot, where they can hide in their own cardboard box or tin garbage can.
Characters who are killed in combat are eliminated from the player's squad permanently, unless they are special characters, such as Snake. Special characters also include team mates that are bosses or supporting characters in the story. If a special character's health is reduced to zero, they are sent to the infirmary, in order to recover. The player can also abort the mission.
A variety of methods can be employed to expand one's squad. If an enemy character is tranquilized or stunned, they can be dragged to a waiting vehicle and captured. After a period of game time, the captured soldier will become a member of Snake's team. Also, enemy characters can be dragged to an ally waiting in a cardboard box, where, through the use of a transceiver frequency, they will be ordered to transport the enemy for the player, saving stamina. Alternatively, by accessing the PSP in certain hotspots using the system's Wi-Fi feature, soldiers and even special bonus characters can be recruited. The PSP GPS Receiver can also be used to similar effect. Because some of the player's recruits include former enemy soldiers or personnel, they can walk among the enemy undetected as long as the player avoids suspicious actions.
Another new feature is the surround indicator added to the game's HUD. Similar to the radar in previous titles, the surround indicator allows players to determine the relative proximity of enemy soldiers by the noises they make. The surround indicator is composed of two circles; the outer circle displays the noises made by enemies and inner circle displays noises made by the player himself.
The game also contains a Wi-Fi-enabled multiplayer mode, which is an expansion of the Metal Gear Online mode previously featured in Subsistence. One's performance in the Online Mode may affect their performance in the single player campaign; the player can recruit and trade soldiers from beaten opponents, or vice-versa. Additionally, certain multiplayer options result in recruits being removed from the one's single player roster permanently. In contrast to the console games in the series, the cutscenes that drive the story are not rendered using the usual in-game engine. Instead, they are presented using an animated comic style comprised of hand-drawn artwork by artist Ashley Wood. This style was previously utilized in Metal Gear Solid: Digital Graphic Novel. The game also features voice acting, consisting of returning cast members from Snake Eater and new actors. However, the number of cutscenes and in-game voiced sections are minimal, due to the PSP's UMD storage capacity constraints.
The two escape and Snake makes his way to a communications base, where he attempts to contact his old CO, Major Zero. Instead, he is greeted by his old FOX comrades Para-Medic and Sigint, who reveal that Snake and Zero are being accused of instigating the revolt and that the only way for Snake to clear their name is to find and apprehend the leader of the rebellion, Gene. To complicate matters, Gene has also convinced most of the Russian soldiers stationed on the base to join their side by simply taking over the chain of command of a former Red Army unit, that was secretly stationed inside the Colombian territory. In order to complete his mission, Snake must persuade enemy soldiers to join his ranks because of the scale of his mission.
Snake and his squad defeat the top members of the FOX unit and eventually they make their way into Gene's guesthouse. Snake learns many things on his way. Cunningham was working for the Pentagon and wanted Snake to push Gene into launching a nuke to Russia to prolong the Cold War. Gene was actually aware of this plan the whole time from information from Ocelot. Gene really wanted to launch a nuke at America to destroy the Philosophers and to make his nation of soldiers, "Army's Heaven". Snake destroys an experimental model of the ICBMG (the Metal Gear model) codenamed RAXA and eventually defeats Gene, destroying the ICBMG itself afterwards. After Gene is defeated he gives Big Boss the funds, equipment, personnel, and all other information regarding Army's Heaven, telling him that one day, he will need to use them. On his return home, Snake is awarded for his actions and establishes FOXHOUND afterwards. In the end, Ocelot kills the DCI and takes the documents containing the identities of the Philosophers to "end them". After the credits, Ocelot speaks with an unknown man on the phone. Ocelot speaks of using the Legacy to form the Patriots in place of the Philosophers. Ocelot actually wanted the trajectory data of the nuke to point to the DCI so he would bring the documents right to him. Ocelot requests Big Boss be able to 'join' them (he and the man on the phone) in their new endeavor - becoming the Patriots. This mysterious man, whose identity is hinted as the "man with the same codename as Null is revealed to be Major Zero in Metal Gear Solid 4: Guns of the Patriots.
In Europe, the game was set to be released in April, yet it was delayed for a month. In the United Kingdom, the game could only be released on May 25, 2007, after it was revealed that the required BBFC rating was missing, forcing retailers to send back their stocks. The added features for the European release included new maps for the single-player campaign and multiplayer mode, characters, missions, player careers, as well as a new "Boss Rush" mode.
In Japan, Portable Ops Plus was released on September 202007 in two formats: the expansion on its own and as a deluxe package that includes the original game and the expansion. The North American version was released on November 13, 2007. The European version was released on March 28 2008.
The story mode within Metal Gear Solid: Portable Ops has been removed, and a new single player mode has been added called Infinity Mission, where the player must go through randomly chosen maps, looking for the advance point within the level, using the surround indicator to help them. This adds some extra difficulty, because the surround indicator could show an enemy, or the advance point. Within this mode, there are also kerotan, which make a small croak sound, as well as briefly showing on the surround indicator with every croak. The game places an emphasis on online gameplay, with the modes from the original being used with their + counterparts, such as "Cyber-Survival+" and "Infrastructure+". During online gameplay, rookie players will have a rookie mark, reducing the damage they get, and increasing the damage they give, making online gameplay easier for less skilled players.
The ending theme ("Calling To The Night") was composed by Akihiro Honda and arranged by Norihiko Hibino and Akihiro Honda, with vocals by Natasha Farrow and lyrics by Nobuko Toda. "Calling to the Night" was later featured in Metal Gear Solid 4 as an iPod track, and Nintendo's Super Smash Bros. Brawl as one of the songs played on the Shadow Moses Island stage.
The soundtrack was first released in Japan on December 20, 2006.
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