As Dr. Gordon Freeman, you were last seen exiting City 17 with Alyx Vance as the Citadel erupted amidst a storm of unknown proportions. In Episode Two, you must battle and race against Combine forces as you traverse the White Forest to deliver a crucial information packet stolen from the Citadel to an enclave of fellow resistance scientists.
A necessary expansion with various engine improvements and a ton of mods available. Surprisingly, this game has better graphics than lots of mainstream crap today, at least on NVIDIA cards with SSAO and supersampling enabled. This is why HL2 E2 has so many modifications.
Any way you look at it, Episode Two stands out, even among the Half-Life series, as something special. It may be shorter than the original, but it's a burly experience packed into roughly six hours or so that offers up all the diversity, level design, and thoughtful gameplay we've known while making sure to propel the story forward and leave us wanting more.
Just like HL2 and HL1, the game is great. It has new Physics systems - the train cutscene and else. It may contain bugs, but I haven't found any. The game may lag for weaker computers (like me :D) but still is very playable. ;D