WARNING - Contains SPOILERS
Half-Life 2 takes place 10 years after the original Half-Life, since Gordon Freeman accepts the G-Man's employeement offer. You, Gordon Freeman, Research Scientist, as the player has been put into an unconsious state after entering the portal out of the tram, and have been completely unaware of the events that have come forth during the 10 years been Half-Life and Half-Life 2. "Rise and shine, Mr.Freeman. Rise and shine. Not that I wish to imply you have been sleeping on the job. No one is more deserving of a rest. And all the effort in the world would have gone to waste until...well, let's just say your hour has come again. The right man in the wrong place...can make all the difference...in the world. So...wake up, Mr. Freeman. Wake up and...smell the ashes." - a nice little quote from the G-Man, stating in essence that Gordon Freeman's task of saving the world is once again neccessary and upon him.
So over the period of 10 years, you might expect the world of Half-Life to have dramatically changed. And infact, that is true. The invasion of the border world "Xen's" alien creatures certainly did not end after you destroyed the Xen alien overlord known as the "Nihilanth".
However, all that really did was free the Vortigaunts, the alien slaves of Xen. No, the invasion of Xen, after persisitingly spread about Black Mesa, once the facility was nuked (forever under a radioactive crater), had spread across the world. In super markets, headcrabs materalise in the isles and on the shelves. On train-tracks, Gargauntuas appear. In pools, Ichysaours teleport. In parks, bullsquids attack. The world was doomed
to the inevitable catastrophic apocolypse of humanity. Citizens decided to make shelter in the City, where they assumed they'd be safe from the vile Extraterrestrial, sentient beings.
But they were not quite. A new race had come forth for a new invasion of the Earth, taking complete control. The Combine, who were attracted to Earth by the dimensional rift caused at the Black Mesa Research Facility incident. And eventually, after several years, the Combine
dwindling the Earth and humanity's resources to the oppressive 1984 esque environment known as City 17 in Eastern Europe, implanting their own designed technology and replacing the pure natural oxygen and air of Earth with nearly sufficating Combine air. City 17 is administered by Dr.Wallace Breen, the old administrator of Black Mesa Research Facility, after making a deal with the Combine.
Now, Earth has been reduced to a deslet place of a prisoning environment of so-called "citizens", under control of the Combine Overwatch and prisoned in the Nova Prospekt. Even the surviving once alien slaved Vortigaunts have been pried under Combine control. And guess who's task it is to sort this all out and save the world. Gordon Freeman, you, the player, as the G-Man wishes. Hope you're up to it.
"Welcome! Welcome to City 17! You have chosen or been chosen to relocate to one of our finest remaining urban settings. I thought so much of City 17 that I elected to establish my administration here, in the Citadel so thoughtfully provided by...our benefactors. I've been proud to call City 17 my home. And so, whether you are here to stay or passing through, away to parts unknown...welcome to City 17! It's safer here." - a quote from Dr.Breen. It's his courtesy way of saying "The Combine are your master. You are their slave. I am your leader, and you will do as I say or you will die." But the people of Earth have not given up hope just yet. A small group
of anti-citizen resistance had formed, some people known as Black Mesa survivors from the incident, in this case, Dr.Isaac Kliener, Alyx Vance, Barney Calhoun, Dr.Eli Vance, Dr.Judith Mossman, the Vortigaunt Allies, and a robotic pet named "Dog", most of them are escapees from the Black Mesa incident, some of them working
in disguise with so-called "civil-protection".
Where the story goes from here is up to you and how you play Half-Life 2!