I played the first one years ago and I wanted so badly to play the second one, but sadly I could not because of my video card. Now with a brand new and upgraded one I played it flawlessly. The game is not disappointing at all. You replace Prophet now and you do whatever it's necessary to destroy the Ceph and its infestation on Earth. With each alien you kill you gain some points (catalyst) which can be used to upgraded your armour, stealth and even speed.
First you fight the Cell, but towards the end Cell soldiers can help you in your quest.
The graphics is astonishing and the animations flawlessly. All I can say is buy it and play it if you're a FPS addict like me.