The premise revolves around a special forces operative codenamed Solid Snake who goes into a solo infiltration mission into the fortified state of Outer Heaven to destroy Metal Gear, a bipedal walking tank capable of launching nuclear missiles from anywhere in the world.

The MSX2 version was initially released only in Japan and the Netherlands. A heavily altered port for the Japanese Family Computer and its western counterpart, the Nintendo Entertainment System, was released months afterwards. The original MSX2 version would be ported to mobile phones in Japan in 2004, followed by its inclusion in Metal Gear Solid 3: Subsistence for the PlayStation 2 in 2006, which led to a more mainstream English localization of the original MSX2 game in North America and Europe.

Gameplay

The player must navigate the main character (codenamed "Solid Snake") through various locations, while avoiding visual contact and direct confrontation with patrolling guards. If the player is seen, the game enters the "Alert Mode." In this situation, the player must hide the character in order to leave the mode. The method of escaping varies depending on the circumstances behind discovery:

• If a player is seen and only a single exclamation mark (!) appears over an enemy's head, only the enemies in the player's present screen will attack and an escape can be made by moving to the adjacent screen.

• However, if two exclamation marks (!!) appear over the enemy (or the player triggers an alarm by being spotted by a camera, infrared sensor or by using an unsilenced weapon), reinforcements from off-screen will appear as well. The player can only escape by eliminating all incoming enemies or by simply moving to a different floor.

The player starts the game unarmed, but eventually gains access to a variety of firearms (starting with the Beretta M92F) and explosives and working his way up to machine guns and guided rocket launchers. Ammo and supplies for each weapon are limited, but are easily replenished. Weapons can not only be used to kill enemies, but also to clear obstacles (such as hollow walls or electrified floors). Snake can also use his fists to punch and defeat patrolling enemies. Occasionally if the player defeats a guard with punches without alerting anyone, the defeated guard will yield a single unit of rations or an ammo box that can be picked up.

The enemy base consists of three different buildings, with multiple floors (including basement levels) within them. The player uses key cards and other items to unlock doors or explore new areas. Doors will only open to their corresponding key cards. Bosses also appear throughout the game to interfere with the player's progress.

Information can be obtained by rescuing hostages being held captive within the buildings. After rescuing five hostages, the player is given a promotion, increasing their "Rank" (up to 4 stars max), allowing for bigger ammo capacity and maximum health. However, if a hostage is killed, the player is demoted to the previous rank.