...
or if your brain hurts a little bit:
Gordon Freeman, MIT graduate, chosen for experiment, goes into chamber, experiment goes wrong, horror, headcrabs, aliens, people turn into headcrab zombies, he gets more and more weapons, the military comes in and tries to kill him, and everything else to cover stuff up, bigger aliens, barney calhoun the rent-a-cop helps him, scientists help him, eventually he goes into the alien dimension "xen" to destroy aliens, flying platforms, alien production, alien boss (big flying dude).
At the end, G-man, a strange suited figure that you see throughout the game in strange situations, such as walking in platforms you can't reach, or staring at you from windows you can't get behind, then walking away like he's supposed to be there, talks to you and shows you a whole bunch of stuff and eventualyl says "either work for me, or die." At the end of hte game, you can enter the portal, or not. Not, you get sent to another alien world and tons of alien grunts are seen, giving the impression you die. Do enter it, and the game ends saying something like "Subject: Freeman. Assignement: pending" and the game ends with heavy metal guitar.
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