the experiments that occur in Black Mesa, Gordon's workplace, open an interdimensional wormhole that allows the the occupants of a different dimension known as Xen to enter and wreak havoc for no apparent reason from headcrabs to Vortigaunts (electricity wielders). A secret government operation is executed to destroy the facility, the invaders, and any witnesses. Freeman makes his way to Xen and fights the Nihilinth, who was the creature keeping the portal open and controlling the Vortigaunts that were being held under its control, evidenced by the green collars on the Vortigaunts throughout the game. It is revealed in the military add-on that the military, too, were employed by the G-Man.
The relevance of this plot is twofold: First, in Half-Life 2, the G-Man references how he has tested or trained Gordon in the past for when he would become useful again, suggesting that the Black Mesa Incident itself was entirely orchestrated by the G-Man or his employers. His broken speech pattern, like that of speech software, suggests he is not even human.
Second, the fact that the aliens from the original were the enemies and are now the friends in the sequel is proof that the Vortigaunts - now free of their binding collars - were indeed being forced to fight the humans in the first one. Their otherwise peaceful nature in the sequel is more than simply being written in, the true nature of the Vortigaunts was suggested by the original and is now resolved, deepening the story.