The genesis of stealth gaming is here. This great game got some things wrong, but invented all new things to get right.
Playing as Snake, you must travel to an Alaskan base to stop terrorists threatening to launch a nuclear strike via a Metal Gear, a massive walking tank capable of blasting a nuclear weapon as if it were a mortar shell. These terrorists also have a common link to Snake, who along with possessing animal nicknames have various special abilities come from a special military "enhanced soldier" program. Getting help from several friends along the way, both in person and over your video link to the military, you'll have to find a way to get to Metal Gear and stop it before it can launch its strike.
The biggest positive you can say is that the number of things to do and items to use is a level not seen before in games. Combined with whether you want to use them to take out enemy grunt soldiers or use them to help evade without them detecting you, and you can really shape the gameplay to the way you want to play. The radar screen that lets you see what the enemies and cameras can detect is an interesting feature that lets you see how the enemies move and plan your strategy. The level of detail is astounding, whether looking at a desk to see what's on it, to the sheer number of conversations you can have on nearly any object you can see. It's as if the game designers correctly predicted every possible thing you might want to talk about and included the conversation for you to have.
Aiming guns is a major weak point. It's often quite difficult to figure exactly where your gun is aiming via the overhead perspective you have to shoot from, leaving you shooting a tad too wide and alerting a guard instead of killing him. Obviously the thought that enemies can only see a few feet ahead, but you can see them clear across the room, is a bit of a stretch, but they make this type of game possible. The game also has a highly agitating quirk of often de-equiping your currently selected item after a cut-scene. Not realizing your body armor is no longer on until after you get hit with a bullet is really infuriating. Also, security cameras regenerate at a far too alarming rate. That camera you destroyed in the previous room? When you go back, there's a good chance it will be back, too.
The feel of the gameplay is very good, and everything responds how it should with never any slowdown. A major gripe would be sticking to walls unintentionally when the camera shifts and you end up running up to a wall instead of parallel to it. And some cut-scenes are a tad too long. The graphics and sound are both top notch, and you're not going to find better on the PS1, whether it's the detail of breath coming from your mouth or the howling shriek coming from a radio contact when you die. The whole game won't take more than about 15 or so hours, and a few hours of that is cut-scenes. Still, this game is so ground-breaking that it deserves to be in your collection, or at minimum to be experienced via a rental.