In the game, Solid Snake must infiltrate Zanzibar Land (unrelated to the non-fictional Zanzibar that is part of Tanzania), a heavily defended territory located in Central Asia to rescue a kidnapped scientist and destroy the latest version of Metal Gear.
Metal Gear 2 builds upon the stealth-based gameplay of its predecessor. As in the original Metal Gear, the player's objective is to infiltrate the enemy's stronghold, while avoiding detection from soldiers, cameras, infrared sensors and other surveillance devices. The biggest change in the game was done to the enemy's behavior. Whereas the guards in the original game could see in straight lines, the guards in Metal Gear 2 all have a field of vision of 45 degrees. The guards can also turn their heads left or right to see diagonally and move from one screen to another (instead of remaining stationed in area). The enemy can also hear any noise made by the player, which usually occurs when the player fires a non-silenced firearm, uses an explosive, or walks over certain types of terrain. If the player is discovered by the enemy, then a counter will be displayed on the upper right side of the screen that will go down after the enemy has lost track of the player. When the counter reaches zero, the alert phase will go off and the game will return to normal.
The player has been given a variety of new maneuvers and tools to help them remain undetected and complete the game. For example, the player can now kneel and crawl in addition to walking, allowing the player to avoid making noise, pick up land mines, and hide under tight spaces such as under desks or inside air ducts. A radar with a 3x3 grid on the upper right of the screen shows Snake's current position in the center screen (as a red dot), with enemy soldiers as white dots, allowing the player to determine what's ahead. However, the radar is disabled when the player is in alert phase. The radar can also be used with the mine detector equipped to determine the locations of any placed mines or fire stinger missiles on an airborne target. Many of the weapons and equipment from the first game are brought over as well as new items such as robotic mice used to distract enemies, a camouflaged mat and three different types of rations with their special attributes. The player no longer needs to rescue hostages to increase their rank; instead their life energy and carrying capacity is increased each time they defeat one of the game's bosses.
The transceiver has been greatly revamped from the first game as well. The messages the player receives are now based on their current situation and mission objective, rather than the room they're currently in, making them less fixed and more dynamic. The transceiver now displays Snake's face, as well as the face of the character he's currently communicating with. The player can also talk to children living in the fortress to gain new information; the player is penalized with loss of health if he kills a child. The game's areas are more varied than in the first game and a number of puzzles must be fulfilled to complete the game, such as luring a carrier pigeon with a ration, chasing after a female spy to the women's bathroom or deciphering secret Tap Codes to gain vital information such as a new frequency number.
Solid Snake, the formerly retired FOXHOUND agent returns as the playable character. His new mission is to rescue the kidnapped Czechoslovakian biologist, Dr. Kio Marv, from the forces of Zanzibar Land. He is assisted by a radio support crew consisting of Roy Campbell, his new commanding officer; McDonnell Miller, a survival coach and drill instructor; George Kessler, a military strategist; and Johan Jacobsen (Yozef Norden in the original MSX2 version), a wildlife expert. Also assisting him on-site are Holly White, a CIA agent posing as a journalist; Gustava Heffner (Natasha Marcova in the MSX2 version), an StB agent and Dr. Marv's bodyguard; and Dr. Drago Pettrovich Madnar, the Metal Gear designer from the first game, who was captured along with Dr. Marv. Also appearing in the game are Big Boss, FOXHOUND's commander-turned-traitor from the first game; and Gray Fox, who disappeared following the events of the first game.
The bosses of this installment consist of Black Ninja (Black Color in the MSX2 version), an experimental drug-enhanced soldier (who is revealed to be Schneider from the original Metal Gear); Running Man, an agile mercenary; Red Blaster, a grenade throwing expert; the Four Horsemen (Ultrabox in the MSX2 version), an assassination squad specializing in confined spaces; Jungle Evil (Predator in the MSX2 version), a jungle warfare expert; and Night Fright (Night Sight in the MSX2 version), an assassin who uses a state of the art stealth suit that renders him invisible.
Oil reserves were at a critical low, and the world community was prepared to take drastic measures, either by drilling into sand and shale for more oil, despite the difficulty -- or moving on to renewable fuels.
Such steps proved unnecessary when Czech scientist Dr. Kio Marv successfully bio-engineered a new species of algae, OILIX, that could produce petroleum-grade hydrocarbons with little expense and effort. Marv presented the algae to the World Energy Conference in Prague, and was on his way to a demonstration in the United States when he was kidnapped by soldiers from Zanzibar Land. NATO discovers that Zanzibar Land's leaders plan to hold the world hostage by controlling the supply of oil and nuclear brinksmanship, via a stockpile of nukes raided from nearby missile sites.
Solid Snake is brought out of retirement by FOXHOUND's new commander, Roy Campbell, and is sent to Zanzibar Land to rescue Dr. Marv.
On the course of his mission, Snake teams up with Holly White, a CIA operative posing as a journalist, and Gustava Heffner, an StB agent and Dr. Marv's bodyguard. He is also reunited with Dr. Drago Pettrovich Madnar, the Metal Gear inventor from Outer Heaven, who claims to have been once again kidnapped and forced to work on another Metal Gear project (named Metal Gear D) for Zanzibar Land, as well as oversee mass-production of smaller, non-nuclear-equipped Metal Gear units. Snake learns from Dr. Madnar that Big Boss, Snake's former FOXHOUND commander, has survived and is the man in charge in Zanzibar Land.
Upon reaching the suspension bridge leading to the Detention Camp, Gustava is killed by a missile fired by Metal Gear D and Dr. Madnar is recaptured by the enemy. The new Metal Gear's pilot is revealed to be none other than Snake's former mercenary buddy Gray Fox. Determined to finish his mission, Snake fights against Zanzibar Land's elite mercenary force and manages to reach Dr. Marv's cell.
Snake arrives too late, unfortunately, as he finds the corpse of Dr. Marv, who was unable to survive the repeated torture. Dr. Madnar is also there and explains he was unable to save him. Just before Snake could retrieve the OILIX formula left by Dr. Marv, Holly reveals via a radio message that Dr. Madnar was not captured after all, but voluntarily resumed his work on Metal Gear and was personally overseeing its development as a result of being rejected by the scientific community after the Outer Heaven incident. After the truth comes out, Dr. Madnar attacks Snake, but fails in killing him.
With the OILIX formula in Snake's hands, Snake is confronted by Gray Fox piloting Metal Gear D. After successfully destroying Metal Gear D, Snake finds himself in a minefield. The two engage in hand-to-hand combat against each other, and Snake eventually emerges as the victor.
Upon defeating Fox and retrieving the OILIX formula, Snake is met by Big Boss while trying to escape. Having lost his equipment and with no weapons at his disposal, Snake is forced to improvise using the only items he can find, a lighter and aerosol can. Fashioning a makeshift flamethrower, Snake defeats Big Boss for the second time. Snake and Holly escape together, and they deliver the OILIX formula to Campbell.
According to Kojima's account of the events, he did not have any plans to design a Metal Gear sequel at the time and was unaware that a sequel was being produced until he became acquainted with a member of the Snake's Revenge development team on a train ride in Tokyo. Kojima was then informed about the development of Snake's Revenge and was told "it's not the authentic Snake, so please create a new Snake game of your own." After being given the go-ahead by his bosses at Konami, he began developing Metal Gear 2: Solid Snake with the MSX division of Konami. Metal Gear 2 serves as a follow-up to the original Metal Gear, ignoring the events of Snake's Revenge (which was unreleased in Japan), and every canonical Metal Gear title released afterward acknowledge only the events of Metal Gear 2, relegating Snake's Revenge to non-canonical status.
A version of Metal Gear 2 for mobile phones was released in Japan on October 1, 2004 for the i-mode, EZweb and Vodafone services prior to the release of Metal Gear Solid 3: Snake Eater. This was the first time the game was ported to another platform since its original release for the MSX2 in 1990. In 2005, Konami released an expanded edition of Metal Gear Solid 3 subtitled Subsistence, which included, among other extras, the original MSX2 versions of Metal Gear and Metal Gear 2. The release of Subsistence in North America and the PAL region marked the first time Metal Gear 2 received an official English localization.
Both re-releases feature several subtle differences between them and the original MSX2 version. The biggest change was in the game's artistic presentation. The character portraits used in the re-releases, instead of the realistic style used in the MSX2 version, features an art style similar to Yoji Shinkawa's character designs in the later Metal Gear Solid games. Numerous characters were renamed as well, notably Natasha Marcova became Gustava Heffner and Dr. Petrovich Madnar became Dr. Drago Pettrovich Madnar. Nearly all the bosses were renamed as well. The re-releases also feature two selectable difficulty settings ("Original" and "Easy"), a bonus Boss Survival mode and an infinite bandana that provides Snake with unlimited ammo when equipped.
The PlayStation 2 version of the game, much like the MSX2 original, requires the player to use the instruction manual for reference to solve certain puzzles (such as deciphering Tap Codes or learning a certain frequency number). Because the North American version only came with a thin manual that did not include the information the game asks for, Konami posted the solutions to those puzzles in a FAQ page on their official website.