A must-kill for dedicated fans of single-player "first person shooter".  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 The Aftermath that matters.)  Oozing high-tech, the facility is host to the denizens of 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.