The following summary is adapted from the "Previous Operations" option in the "Special" menu of MGS1:
The year was 1995. Deep in South Africa, 200km north of Galzburg was Outer Heaven - an armed fortress nation established by the legendary mercenary. He was feared by both his friends and foes as a hero and lunatic.
The "Western" nations found out that a weapon of mass destruction capable of rewriting war history was under development at Outer Heaven. They called upon the high-tech special force unit FOX-HOUND to take care of the situation. In response to this order, Big Boss, commander-in-chief of FOX-HOUND, sent Gray Fox, the man with the codename "Fox", which is given to the best member of the unit "Operation Intrude N313".
After a few days, his last message being "Metal Gear...", Gray Fox was declared missing in action. Taking the situation seriously, the top men of the "West" again called for FOX-HOUND. Big Boss selected Solid Snake, who had just recently joined FOX-HOUND, as the agent, and entrusted everything to him.
Successfully making a solo infiltration of Outer Heaven, Snake got in touch with local resistance members Schneider, Diane, and Jennifer. With their co-operation, Snake succeeded in rescuing Gray Fox. Gray Fox then laid out the terrifying facts about Metal Gear. Metal Gear was the development name of a nuclear warhead-equipped two-legged walking tank. It could walk through even the roughest terrains that would stop normal tanks. It could conduct local warfare by itself with unique weapons like its Vulcan cannon and anti-tank missiles. It was indeed a new-type of weapon that could conduct a nuclear attack against any place on the face of the earth from any land surface.
With Metal Gear, Outer Heaven was trying to establish its military superiority over the entire world. In order to destroy Metal Gear, Snake rescued Metal Gear's chief engineer Dr. Pettrovich and his daughter Elen, who was taken hostage to force her father to continue with his development. Snake learnt from Dr. Pettrovich how to destroy Metal Gear. However, as Snake approached the heart of Outer Heaven and Metal Gear, he discovered well-designed traps set out all around him - almost as if all his actions were being leaked to the enemy.
In the midst of the escalating battle, the leader of the resistance, Schneider, fell into the hands of the enemy, and Snake himself was injured through the deadly battles with Outer Heaven's best mercenaries.
But Snake's indomitable spirit lead him to the 100th floor basement of the secret base where Metal Gear was developed. Evading the powerful defence system that could wipe out all intruders, Snake ultimately succeeded in destroying Metal Gear.
Snake tried to escape from Outer Heaven upon completing his mission. However, during the escape he was confronted by one man - FOX-HOUND commander-in-chief, Big Boss. Big Boss laughed at the astounded Snake and told him of the truth about his mission.
While serving as the commander-in-chief of FOX-HOUND, Big Boss also ran a mercenary dispatch company utilizing his connections and capital from his years as a mercenary.
He had planned to build this company a larger military establishment, and he had Outer Heaven built as its base. His purpose for sending the rookie Snake to Outer Heaven was to cause information confusion against the "West".
However, Big Boss miscalculated. He never thought that Snake would make it this far.
Having lost Metal Gear, Big Boss activated the self-destruction system of the underground base. While the countdown to destruction continued, his scream echoed in the emptiness "You have gone too far. Too far!"
On the 100th floor basement, the battle between the two men commenced - free of ideology and politics...
The armed fortress nation, Outer Heaven, collapsed. The impenetrable fortress made from the best military technology, and occupied by the toughest mercenaries, burned in flames. Behind him, the flames reached skyward, as Outer Heaven fell, leaving Solid Snake all alone...