Rent it or you'll have to buy the previous one.
Your some kind of mercenary or something and you work for various people. The point being you won't pick up this game for the story but you shouldn't really pick it up at all. All the enemies are better than you and its not based on skill it's just based on how utterly weak you are and if you could make it good for you. This game is punishing but doesn't keep you wanting more because it is just unfair to the player. If you own the previous iteration armored core 2 and did well you can transfer over your stuff.
I believe that is what this game was really intended for people who bought the previous version because you can't go very far without it. It was a nice try though. I had fun for the portions of the game I could play.
I extremely disliked the fact that I would have to buy a previous version to enjoy this one so I sold it back.
There is multiplayer but it isn't that fun it isn't online either so you'll need a friend that will play with a bunch of random mechs you've taken the time to design or the general "bad" ones. The maps are few and pretty dull too.
Graphics were ok. My biggest issue again was with the gameplay the controls were okay but for the most part delayed as your mech can't turn that quickly while everyone else's can and they dance around you as you "try" to aim at them.