Half Life 2 is about is about a man named Gordon Freeman who was pretty much captured and is now in this city that is controlled by a bunch of aliens called the combine. from here you fight your way through the combine and other monsters to meet up with old friends (Half Life 1) to help destroy the Citadel which Dr. Breen (Half Life 1 character) is trying to take over the whole place and stuff like that.
great game i would most definitely buy it. if you are a serious gamer, i suggest picking up a copy of the first Half Life. i'm just saying this because there are a lot of characters and monsters from the 1st one in the 2nd one. but Half Life 2 can basically be catorgorized as its own game and not a sequel because it barely referres to the 1st Half Life, besides the characters and monster part what what ever