Metal Gear is considered to be the progenitor of the stealth game genre, it was also the first video game designed by Hideo Kojima, who also directed many of the later Metal Gear sequels. The game 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 player must navigate the main character, a military operative 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, Snake must escape from enemy's sight in order to resume infiltration. The method of escaping varies depending on the circumstances behind discovery: If Snake 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 Snake can escape by simply moving to an 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 unsuppressed weapon), reinforcements from off-screen will appear as well. Snake can only escape by eliminating all incoming enemies, going outdoors, or by entering an elevator. The player starts the game unarmed, but eventually acquires firearms such as a Beretta 92F and an M79 grenade launcher, as well as explosives such as remote control missiles and landmines. Ammo and supplies for weapons are limited, but are replenished by picking up ammunition caches or additional supplies. Weapons can not only be used to kill enemies, but also to clear obstacles such as hollow walls and 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. In addition to enemy guards, the player will also be confronted by mercenaries who will challenge the player to combat, serving as the boss characters. 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 and explore new areas. Each door will only open to its corresponding key cards. Information can be obtained by rescuing hostages being held within the buildings. After rescuing five hostages, the player's rank will increase by one star (with the maximum rank being four stars), allowing for increased carrying capacity and maximum health. However, if a hostage is killed, the player is demoted to the previous rank. A transceiver is available for the player to communicate with their commanding officer, Big Boss, or one of the resistance members operating covertly near Outer Heaven (Schneider, Diane, and Jennifer). Each of Snake's allies specializes in a specific subject. The player must keep track of their allies' frequency numbers in order to contact them. All of the radio messages are dependent on the rooms where the player is located.

Near the end of the 20th century, the West discovers that a weapon of mass destruction is being constructed inside Outer Heaven, a fortified state founded by a "legendary mercenary" 200 km north of Galzburg, South Africa. The special forces unit FOXHOUND sends top agent Gray Fox to infiltrate the fortress, assess the situation and neutralize the threat. FOXHOUND loses contact with Gray Fox a few days later, with his last transmission being "METAL GEAR... " To discover what happened to Gray Fox, FOXHOUND commander Big Boss sends his newest recruit, Solid Snake, into the area in an operation codenamed Intrude N313. Upon insertion into Outer Heaven, Snake makes contact with local resistance members Schneider, Diane, and Jennifer. Using all of his skills and the equipment he procures on site, he manages to rescue Fox. Fox explains that Metal Gear is the codename of a nuclear-equipped bipedal walking tank, which can engage in all forms of combat and launch nuclear weapons from any location. Outer Heaven plans to use Metal Gear to impose itself as the new world superpower. To destroy Metal Gear and topple the Outer Heaven mercenaries, Snake rescues lead Metal Gear engineer Dr. Pettrovich Madnar and his daughter Ellen. The scientist explains how Metal Gear can be destroyed, and Snake takes on Outer Heaven's troops. However, he begins to notice that traps put in his way are too precise, and wonders how information on his activities are being tracked. Big Boss begins to act strangely and eventually tells Snake to abort the mission (breaking the fourth wall by telling the player to turn off the console). Moreover, Schneider is ambushed by hostiles and is presumed dead after losing contact with Snake. Snake penetrates Outer Heaven's main base and takes out Metal Gear before it reaches completion. As he safely escapes the compound's basement, he is confronted by the mercenary leader of Outer Heaven, who turns out to be Big Boss. The corrupt leader reveals that he had been using his connections to steal military intelligence, establish his own mercenary force, and fund his activities. It was his aim to have Outer Heaven become the world's greatest superpower, able to bring even the West to its knees. He had the rookie Snake sent in, hoping to have him captured and feed misinformation to authorities, but had quite obviously underestimated Snake's capabilities. Having lost Metal Gear and much of his force, Big Boss seemingly starts the self-destruct sequence for the compound, and promises he will not die alone; Snake will join him. Snake defeats Big Boss in a last battle and escapes the Outer Heaven compound as it crumbles in flames behind him. After the end credits, a message from Big Boss is displayed saying that he will meet Solid Snake again.