As with everything else to do with the game, Valve is keeping HL2's storyline heavily under wraps. However, also as with everything else in the game, many sites have attempted to piece together a viable storyline from interviews, screenshots, and leaked information.
THE REAL STORY
Here's what I've maqnaged to discern. The Earth has been invaded by aliens that you fought int he first game, and so the majoirty of the population on Earth has relocated to Eastern European military cities (hence, the main setting of the game, City 17). Now working for the insidious G-Man that you can face to face with on numerous occasions in the first game, you play Gordon Freeman once again, this time entering a different chapter in the story.
Valve has said that the entire game will be played on Earth, and that there will be no return to any aliaen worlds. Of course, this could all be some strange marketing ploy, but then again everything else might be as well
