I've just played through this (game seems a tad short at under 7hrs? And I'm quite a slow gamer...). Here's my take on the story:
This alien race, the shroud, have taken your daughter hostage, and are forcing you to deliver the virus (that woman) to them. However this isn't known until the last level, it's supposed to be some kind of plot twist - up until that point, you are supposed to just be thinking about helping her to escape. As for the final sequence, basically your character commits suicide, because he knows that if he dies, it will cause a huge explosion, thus killing all remnants of the virus (his infected blood, and the woman). In an earlier scene you hear that he has basically given up hope of getting his daughter back as she was, so he is willing to sacrifice his own life for the good of humanity (by wiping out the virus).
The problem with this game, is that the story is very random, with no clear logic to it's progression. You are plunged into the game with very little idea of what is going on, and the story develops in fits and starts. The trouble is that it often has very little to do with the action, I mean take "The Anvil" level. Why, exactly, are you trying to get inside the Anvil? Apart from the sections where you are trying to find the woman, most the story and action seem severely disjointed. I mean take the weapon that suddenly appears on the final level, wtf is that all about? Where did it come from, why weren't you using it earlier, why have all my other weapons disappeared? At least in HL2, there is some kind of story behind the super gravity gun.
Viewing the game overall, it's hard to recommend this. The story is weak, and the action average at best - it's kinda like Halo, only shorter, with better graphics. The weapon system is irritating, with ammo packs getting wasted because you didn't have a full clip (including background weapons) when you pick them up.
The enemy does make use of cover, to be fair, and there's a suprising amount of interactivity with the environment - the physics work quite well. But there's too few enemy types and the combat feels too samey. The weapons you use the most - machinegun, shotgun (I forget the real names) and plasma rifle are unfortunately pretty poor implementations. The sniper rifle and grenade launcher are cool, but you don't get many opportunities to use them (I didn't fire a single missile, despite upgrading it to quad-rox, so I can't comment on that). Weapons on vehicles are wildy inaccurate, often the best way of killing people is simply to run them over. So all-in-all, combat is rather unrewarding.
If this game was a bit longer, one could forgive it's deficiences, but as it stands, it feels woefully outdated. This is 2006, not 2002. It's obviously focussed primarily at consoles, with the checkpoint save system and option for auto-aim, and seems a bit easy on normal difficulty. It could do with having 25-30 levels instead of 18, and a bit more production on the story side of things.
Edit: At least performance is good
