You are Gordon Freeman, a young research associate in the Anomalous Materials Laboratory. You have limited security clearance and no real idea of just how dangerous your job has become, until the morning you are sent alone into the Test Chamber to analyze of a strange crystalline specimen. A routine analysis, they tell you. Until something goes wrong. 

Is it sabotage? An accident? Or is it something you did? All you hear is screaming; all you see is spacetime shattering. The next thing you know, the entire Black Mesa Facility is a nightmare zone, with sirens wailing and scientists fleeing in terror from the things their co-workers have become. 

Hordes of creatures from the far side of the portal are pouring through rifts in the local fabric of reality. Monsters are everywhere. Madness rules. You head for the surface, but the usual routes are impassable-closed off by the disaster, infested with headcrabs and houndeyes and increasingly larger and hungrier creatures. 

As Gordon Freeman, you must enlist the help of traumatized scientists and trigger-happy security guards to get through high-security zones, sneaking and fighting your way through ruined missile silos and Cold War cafeterias, through darkened airducts and subterranean railways. When you finally come in sight of the surface, you realize that the inhuman monsters aren't your only enemies-for the government has sent in ruthless troops and stealthy assassins. Their orders seem to be that when it comes to the Black Mesa, nothing gets out alive... and especially not you. 

When even your own species turns against you, maybe you'll be glad to see another portal beckoning. But then again, on Earth you have allies; while on the far side of the portal, nothing at all is familiar except the sense of danger. 

Save the Earth? Well, maybe. But that's a pretty low priority compared to saving your own skin.