After the Black Mesa incident... Save your own skin from the military and creatures warping in from Xen.
Its a short fun ride through some good level design, fast paced action, some nice areas and a ending boss that you're not expecting.
The levels are nothing special in the beginning, very normal textures, some headcrabs and zombies... its only until the tram ride where things start looking end, before quickly going back to drab.
I would have liked for there to have been something more to the mod in terms of storyline or something amazing gameplay wise, but it doesn't try to be anything more then just what Half-Life in terms of gameplay just without the big battles and vent crawling.
Its patch adds some human grunts which is where some of the good fights come from.
and while its not original (uses the blue shift ending) it's decent enough and stays true to what HL was the majority of the time.
The second one ups the ante and delivers some of the best level design I've seen in mods, its goes straight to the action one of the advantages of being a sequel and provides unique ending which gets its idea from what happened at the end of Opposing Force. There are some great set pieces and big fights.
Of course there where some parts that lacked detail but they where very few and far between.