After the Black Mesa incident... Save your own skin from the military and creatures warping in from Xen.
It was a really fun mod. It really captured the elements that the first Half-Life games were all about. I liked how in the end, when you were waiting for the portal to work, a robot came in and started shooting. This was one of the few mods I have played that have a real ending boss, and one of the VERY few I have played where the bosses made me feel intimidated. I was trying to hide from it most of the time. The voice acting is hard to understand, but that is one of the few things that are wrong with this mod.