In the year 19XX, the militarized microstate, Outer Heaven, kidnapped a weapons scientist, Dr. Pettrovich. He was coerced into developing weapons for the nation, among them TX-55 Metal Gear.
Eventually, rumors of Outer Heaven's development of an ultimate weapon's leaked to foreign nations, alarming the Western Bloc. In response to this, Special Forces Group "FOXHOUND" was deployed to investigate the threat.
FOXHOUND's first foray into Outer Heaven was conducted by Gray Fox. He was able to confirm the existence of the ultimate weapon, Metal Gear, and managed to tell FOXHOUND some information. However, he was captured by the enemy before he could inform FOXHOUND of its nuclear capability.
Assuming that Fox's final transmission indicated the existence of the ultimate weapon, FOXHOUND began their next attempt at infiltrating the fortress nation, Operation Intrude N313. The mission's goal was for Solid Snake, a rookie operator, to find Fox, confirm the existence of Metal Gear, and destroy it.
After being captured by Outer Heaven, Snake was thrown into the brig. There, he met Fox who informed him of Metal Gear's nuclear capabilities, as well the location of its creator, Dr. Pettrovich.
Snake made his way to Pettrovich, who informed him of Metal Gear's location: the 100th floor basement of Outer Heaven's Building 3. He also told Snake that Metal Gear had a weak point: its thinly-armored legs, which could be easily damaged by plastic explosives. By applying the bombs, sixteen in total, in a specific order and detonating them one-by-one, the legs could be destroyed, crippling the entire craft. However, Pettrovich forgot which leg the final bomb must be placed on.
Having discovered Metal Gear's location, Snake entered its hangar. With the pilotless mecha guarded solely by lasers, Snake successfully placed the explosives on its legs, and correctly guessed the ending to Pettrovich's sequence. After the final bomb was detonated, Metal Gear collapsed and exploded.
"Metal Gear is a nuclear-armed, heavy walking tank adaptable to any terrain. When complete, it enables nuclear attacks from any location on Earth."
—Gray Fox
TX-55 Metal Gear had a height of 6.05m, a width of 2.75m, and a length of 3.25m. It weighed 20.5 tons, and had a walking speed of 50kph. Its armor, variably constructed of tungsten and a special heat-resistant ceramic, was between 30 and 110 millimeters thick across the craft.
Metal Gear's design was very dense: its cockpit, camera head, main cannons, missile launcher, were packed together in close proximity on the mecha's torso. The machine possessed no arms, but had stout legs.
The mecha's main feature was its massive IRBM launcher, mounted on its right shoulder, which allowed it to be equipped with up to two missiles. Due to the missile launcher's immense weight, the mecha's left side contained a large counterweight intended to keep the vehicle's balance during locomotion. A few exhaust pipes were also built into the missile launcher.
The cockpit was conversely located on Metal Gear's left shoulder. It could carry a single crew member, the pilot, and contained the mecha's computer systems, whose large heatsinks were exposed on the craft's roof, along with the nearby multi-antenna module. The hatch was armored, giving it a slightly distinct coloration from the rest of the cockpit. Beneath the cockpit was a large floodlight.
The machine's "head", its camera eye, was located between the missile launcher and cockpit. It contained three sights, including a "TV sight", which likely interfaced with the antenna module, presumably allowing for nearby allies to see from the mecha's POV.
Metal Gear was armed with three guns for ground combat: two single-long-barrel 15mm automatic machine guns, and one flat-barrel 20cm laser cannon. The guns were arranged in a trifecta. It's possible that the entire fixture has the ability to spin, as the guns are depicted in various arrangements.
"In order to conduct an accurate nuclear launch, Metal Gear extends a stabilizing leg from its rear and anchors it to the ground before firing."
Metal Gear's unique reverse-jointed legs allowed it to become an all-terrain tank; however, they were also its main weak point. While its feet were protected, the legs themselves were thinly-armored, with many of its components including its hydraulic suspension fully exposed. The only armor the legs possessed were twin tassets, which could fold down and protect the upper thighs, but this only afforded protection from the front.
Metal Gear also possessed an early warning system, consisting of two sensors which covered its main blind spots. Attached to the side of the missile launcher was a radar device for detecting threats from above, and there was also a pulse sensor mounted to the rear of the cockpit which detected threats from behind.