you are gordon freeman, a scientist working at Black Mesa. you are currently working on teleportation when something goes terribly wrong and portals to otherworlds open and aliens come to earth. durring the game your objective is to get out of the black mesa fasility to tell the world what has happened.
Doesn't it seem like they ripped this right off of Doom?
Doom's backstory, as outlined by the manual (reproduced by Wikipedia):
QUOTE[...]Suddenly, something goes wrong and creatures from Hell come out of the teleportation gates, or "Gateways". A defensive response from base security fails to halt the invasion, and the bases quickly get overrun by demons; all personnel are killed or turned into zombies. At the same time, Deimos vanishes entirely. A UAC team from Mars is sent to Phobos to investigate the incident, but soon radio contact ceases and only one human is left alive — the player, whose task is to make it out alive.