The following summary is adapted from the "Previous Operations" option in the "Special" menu of MGS1:
1999 - the world was facing an energy crisis. It was obvious that the supply of petroleum would run out faster than expected. However, the development of an alternate energy resource was far from completion. The price of petroleum had skyrocketed and the world economy was in confusion. The 21st century was expected to be one of chaos. One man's invention changed the entire situation. A Czech genius and biologist, Dr. Kio Marv, invented "OILIX" a micro-organism that could refine petroleum to produce a highly purified form of it.
The world filled with hope upon the discovery of this messiah to solve the energy crisis, but at the same time the world entered a time of tension regarding this new algae. Just when the whole world's attention was drawn to OILIX and Dr. Marv, he was abducted by someone and disappeared. Nations began investigations immediately, and name soon appeared..."Zanzibar Land".
Zanzibar Land was a democratic military regime that suddenly appeared in Central Asia in 1997. When their uprising took place, the CIS army, formed around Russia, sent a suppressive unit immediately. Zanzibar Land resisted by gathering a band of mercenaries from nations around the world and fortifying most of its land.
As a result, the CIS army was repeatedly defeated, and Zanzibar Land declared its independence. Due to the active role mercenaries played, this war was called The Mercenary War, and Zanzibar Land was referred to as an armed fortress nation. A military nation with a good group of strong mercenaries, surrounded by a tough fortress. According to the latest information, Zanzibar Land supposedly was armed with nuclear weapons.
The whole situation was crystal clear. By obtaining OILIX with nuclear weapons, Zanzibar Land was trying to establish its economic and military superiority over the entire world.
Concerned about the situation, the United States ordered Roy Campbell, commander-in-chief of high-tech special forces unit FOX-HOUND, to rescue Dr. Marv. Campbell was a former member of FOX-HOUND. He brought back Solid Snake, the man who had single-handedly brought down the armed fortress nation of Outer Heaven four years previously and asked Solid Snake to bring back Dr. Marv and OILIX. Successfully infiltrating Zanzibar Land, and with the help of CIA agent Horry and others, Snake was able to go deep into the fortress and meet again with Dr. Pettrovich, the chief engineer of Metal Gear at Outer Heaven. He too was abducted to Zanzibar Land and was forced to develop another Metal Gear. He told the surprised Snake of an even more shocking fact. Big Boss, the man Snake had defeated at Outer Heaven, turned out to be the general commander of Zanzibar Land.
Snake rescued Dr. Pettrovich in co-operation with Natasha, Dr. Marv's guard and former Czech International Secret Police agent, and then headed for the confinement facility deep in the fortress to save Dr. Marv.
When Pettrovich and Natasha crossed the narrow suspension bridge over the deep valley, a missile blew the bridge away. Natasha flew into the air because of the explosion. While unable to do anything to save Natasha, Snake started hearing a well known voice.
"Hey Snake. We're good buddies. I can let you go. Just leave this place at once!"...Gray Fox! Snake saw Gray Fox controlling Metal Gear.
The best soldier in FOX-HOUND who after the fall of Outer Heaven, disappeared as if he had followed Big Boss. Losing Natasha in front of his eyes and letting Pettrovich get taken away, Snake screamed, "Fox! I will not give up!"
After a series of deadly battles with mercenaries, Snake finally made it to Dr. Marv's confinement facility. However, he arrived too late. He saw Dr. Marv's corpse and Pettrovich, who could do nothing but stand there. Pettrovich told Snake that Dr. Marv could not withstand the repeated tortures because of heart problems. Snake then received an emergency call from Holly. The information she supplied was very shocking. Pettrovich had voluntarily visited Zanzibar Land to develop Metal Gear.
The abduction of Dr. Marv was conducted under the directions of Pettrovich himself. After the truth came out, Pettrovich attacked Snake, but Snake easily put an end to him and obtained the structural plan of OILIX.
While trying to escape, Snake was confronted by Metal Gear again, controlled by Gray Fox. The tremendous battle took place in the underground base. Snake finally succeeded in destroying Metal Gear. However, Gray Fox did not submit and challenged Snake to the final battle.
In the midst of a minefield, Snake and Gray Fox fought without any weapons. A fist-to-fist duel involving no hatred or murder intent. During that weird moment of purity, the two men were bound by forces transcending words and emotion. Snake won the tough but pure battle against Gray Fox. However, there was still someone else Snake had to fight - Big Boss. Just like four years ago at Outer Heaven, Big Boss was waiting for Snake.
"One who has experienced the tension of battle can never leave the battlefield. I am the one giving you something to live for, and that is war". Snake was infuriated at the arrogant Big Boss. "There is only one battle I have to fight. To free myself from you, to shatter the nightmare...Big Boss, I will kill you!"
With the structural plan of OILIX, Snake and Holly escaped from Zanzibar Land on a rescue helicopter. Snake had once again saved the world. However, there was no smile on his face. Big Boss' last words kept ringing in Snake's head. "Whoever wins, our battle does not end. The loser is freed from the battlefield, but the winner must remain there. And the survivor must live his life as a warrior until he dies".
Snake then disappeared into the white lands of Alaska, alone...