The premise of the game is that your girlfriend is dead and you want to bring her back to life because you can't deal with it I guess. Anyway, you steal a magic sword and trek to some forbidden land which has sixteen huge thingies (though a couple of them are less than impressive in size) that some crazy voice tells you that you need to kill to get your girlfriend back. The voice is Dormin, by the way, some apparently evil thing, though his motives are unclear until the end of the game (which I will spoil later).
Anyways, the entire game consists of riding your horse (Agro) great distances, blindly searching for the next colossi (you can use your sword to get a general direction, but only in the sunlight) and then figuring out how to climb said colossus and strike his weak point (which is one or more glowing seals that you need to stab with your sword a bunch of times) for massive damage.
Once you figure out how to climb the colossus (or get him to reveal his seal) the game is pretty simple. Just hang the fuck on and wait for him to stop flailing around so you can stab him. With a couple of them (especially the last one) it gets downright tedious doing this. But overall, it's a pretty good game... except the ending... here's where the spoilers come in! Don't read the rest of this article if you don't want to be spoiled, okay? So essentially it turns out that Dormin was split into sixteen parts and each piece was enclosed within a colossus or something like that. The reason for the existence of the colossi is never given, so I assume this is their only reason. Anyway, when you kill a colossus (which is really sad, by the way) it frees part of Dormin. Once all sixteen are killed then you see the end of the game where the high priest and his buds show up and yell at you and then stab you in the heart for being a jerk and also for being 'possessed by the dead'. You pull the sword out and I guess you die and Dormin takes over your body.
Next you have to smash the shit out of the high priest, except he escapes and tosses the magic sword into a pool of water which makes weird lightning that sucks you and Dormin in. You can't escape from it and you die again.
Then your girlfriend wakes up and your horse (who falls off a cliff before you meet the last colossus) shows up with a broken leg and they wander over to the pool and find a god damned baby in it instead of water. I assume the baby is the player reborn or something. Also he has horns. Anyway, they take the baby to a garden and meet a deer and the game ends.
What the fuck did I just watch?
But yeah, other than the ending that kind of sucked and didn't make a hell of a lot of sense, it's a pretty good game. Worth the $20 it costs now anyways. If you love boss battles but hate all the other crap in games except for riding a horse around, you will love Shadow of the Colossus!