Half-Life 2: Episode One is an expansion that continues the story from where it was left at the end of Half Life 2. You can encounter many of the characters from the previous game--they return with new abilities and storylines. This expansion focuses on the development of characters other than Gordon Freeman. The engine has also been improved, providing dynamic lighting effects that better model how the human eye reacts to different light sources. Gameplay additions will extend to multiplayer mode as well.