Shadow Law said:
All I wanted to know from half life 1 is were the hell did those head crabs come from?
Here, I'll give you the synopsis.
In the first game, you, Gordon Freeman, are a theoretical physicist at the top-secret Black Mesa Research Facility in New Mexico. An experiment goes wrong and opens up a portal to an alien world, through which a number of alien lifeforms - including the headcrabs - start pouring. Eventually, the military shows up to clean up, and does so by killing everything alive that they see - alien or human. Eventually, some scientists manage to isolate themselves in a teleportation chamber and come up with a plan. They send you to the alien world of Xen, where you ultimately destroy a particular alien called the Nhilanth, who controls one of the major species that are pouring through. Throughout the game, you are followed around by a dour-looking man in a blue suit known as "The G-Man." The G-Man teleports you away from the battleground, and offers you a job; exactly what the job is or who you're working for is unclear. You're then left in some sort of 'limbo' dimension until the beginning of Half-Life 2.