I have the PS2 version, so I hope it's a bit different from the x-box version. But anyway, the story is something like the governments around the world focusing on mental powers. They stop this, some people from all over the world start up in secret on their own and turn bad. Only a few good people with mental powers remain, and guess what? You're one of them. This sends you on a solo-mission to save the world or something. It might sound great, but it's really not.
As for co-op, this is indeed an option on the PS2. Or so they say, but I really can't tell what they were thinking when they made it. Even though there are two players, there is only one person on the screen. That's right. And how would that work? Well, one of you makes him walk and aim. The other one shoots and uses the psi-powers. I know what you are thinking: "Huh? Are you kidding?" Sadly, I'm not. It's the worst idea I've heard since someone said "George W. Bush for president!", and it proves how great the rest of the game is. :woot
I would rather recommend Second Sight. It is also about some guy with mental powers, but it's much better. It can be very hard in the beginning, but after a few levels it will be much easier. The story are far better, the characters better, the powers more interesting and so on. The problem is the cut-scenes look like something the cat dragged in, but it's worth it.