Fans of dedicated single-player "first person shooter" games who've never played this game before should definitely not pass it up.  HL takes an utterly unoriginal idea for either games or movies and crafts an utterly unstoppable gaming experience.  You play Gordon Freeman, who works in the bowels of an ultra-secret, underground research facility called "The Black Mesa".  (Having played the game yet I'm still not sure what's supposed to be going on in BM - it's what happens that matters.)  Oozing high-tech, the facility is host to another dimension when you/Freeman accidentally...do something that tears a whole in the fabric of our own - allowing beings from that other world to enter ours.