In the year 1995, 200km north of Galzburg, South Africa, lies Outer Heaven, a fortified state founded by a "legendary mercenary". Word had gotten out to the western world that deep within the Outer Heaven stronghold, a weapon of mass destruction is being constructed. The United States government orders the high-tech Special Forces unit FOXHOUND to infiltrate the fortress, assess the situation and neutralize the threat. In his mission, FOXHOUND operative Gray Fox infiltrated the Outer Heaven stronghold. Days later, contact with Gray Fox is lost and his last transmission simply reads "METAL GEAR..."
Operative Solid Snake, FOXHOUND's newest recruit, is ordered by squad leader Big Boss to rescue Gray Fox and carry out his mission, "Operation: Intrude N313". Upon insertion into Outer Heaven, Snake makes contact with local resistance members Kyle Schneider, Diane, and Jennifer. Using all of his skills and the equipment he procured on site, he manages to rescue Fox. He learns from Fox that Metal Gear is the codename of a nuclear warhead-equipped bipedal walking tank, which can engage and dominate in all forms of combat as well as launch its nuclear warheads from any location. Outer Heaven plans to use Metal Gear to impose itself as the new world superpower.
In order to destroy Metal Gear, and topple the Outer Heaven mercenaries, Snake rescues the lead engineer of Metal Gear, Dr. Drago Pettrovich Madnar, and his daughter, Ellen. Armed with information on how to destroy Metal Gear, his skill and indomitable spirit, Snake fights through the entire Outer Heaven force. He begins to notice that traps being set for him are too precise, and wonders how information on his activities is being tracked. Moreover, Schneider is ambushed by hostiles and is presumed dead after losing contact with Snake.
Big Boss reveals himself to Solid Snake in the game's final battle (MSX2 version).
Upon breaching the core of the base, Snake is able to neutralize Metal Gear before it reaches completion. As he makes his way out of the compound's basement, he is confronted by the mercenary leader of Outer Heaven, which turns out to be Big Boss. The corrupt leader reveals that he had been using his connections within the U.S. government 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 most powerful nation, able to bring even the western nations to their knees. He had the rookie Snake sent in, hoping to have him captured and feed the U.S. government misinformation, but had quite obviously underestimated Snake's capabilities.
Having lost Metal Gear and much of his force, Big Boss starts the self-destruct sequence for the compound, and promises he will not die alone; Snake will join him. An underground battle ensues, and despite injury, Snake defeats Big Boss 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.
