"Wake up, Mr. Freeman. Wake up and smell the ashes."
You awaken on a dirty train. The people around you don't recall seeing you even get on. Regardless, your ride stops, and you leave, only to be greeted by the sight of projectors spewing propaganda and gas-mask wearing guards armed with electrified batons. These are the consequences of your little sleep at the end of Half Life; while you were in stasis, a race known as the Combine came and conquered the planet with little to no effort. Humanity is enslaved, worse, dying out.
Fortunately, like everything else the Freeman has encountered, it's nothing that can't be solved by a crowbar to the face.
Arguably one of, if not *the* best FPS' out there, Half-Life and its expansions combine action and storytelling in a way no other game has been able to emulate. It is truly something that all gamers must experience, and I highly recommend it.