Before the advent of Metal Gear, all action/adventures were quite
straightforward: Go Forth, and Let The Trigger Twitching
And then Metal Gear came. You're still trying to pull the same
stunt? You're dead, pal, you're dead.
Metal Gear Solid's subtitle summarises best what the series is all
about: "Tactical Espionage Action". The hero of the games,
Solid Snake, is a surprisingly realistic action hero: If he takes
couple of bullets, he won't be breathing any longer. And his enemies
are (more or less) equally mortal - except that they're there in
Snake needs to use the trigger, yes, but violence is never the big
factor in MG series. The games are actually about sneaking. Snake is a
covert operative, not an one-man army. The trick is to stay hidden and
kill noiselessly. Thus, the games are extremely tactical. Use eyes,
make decision, and run in the shadows.
"Big Boss here. Your mission is to infiltrate! Be careful
not to let enemy detect you."
- Big Boss,
Metal Gear (Engrish NES version)
And what's "Metal Gear", then? It's a new generation super weapon. Think of a 'Mech with a nuclear weapon as its heaviest armament. Snake's mission is to destroy it (at least in MG and PS/GB versions of MGS.