“You are Gordon Freeman and you get a call from your friend, the scientist Kazus. He workes at the reception of the Black Mesa Explosive Centre, somewhere in the mountains in South America. He says something’s wrong, then you hear some shots and you hear Kazus screaming. You decide to take a look. When you arrive at the reception room, you see Kazus’ corpse. When you step out, you see only one thing: BLOOD AND BONES….. Who did this? You’ll have to find out and you decide to go to the Black Mesa Explosive Centre.”
“Three years ago, my newly founded work associates and I started work on a portal experiment that went horribly wrong, due to “unforeseen circumstances”. Between our world and Xen, a horrifically violent alien world, a large rift in the fabric of space and time was formed. This allowed all manner of violent species to come, uninterrupted, through the portal and vanquish the Earth. They caused the apocalypse as we now know it.”
Pretty much the standard mod premise: You are Gordon Freeman and you need to do what you do best: Keep moving forward, killing pretty much everything in your path.
The mod finishes when you read the text that says “The End”.