Half-Life 2: Episode One is the first of a trilogy of episodes for the 2004 first-person shooter game, Half-Life 2. The episode takes place immediately after the end of Half-Life 2, in and around the war-torn setting of City 17. Players are forced to deal with the effects of their actions during the main game. The episode is a stand-alone game; while a continuation of Half-Life 2, it does not require the original game to be installed or registered to a user's Steam account to play. It takes advantage of several major upgrades to the Source engine since the release of Half-Life 2, primarily its high dynamic range rendering capabilities and the upgraded facial animation system.
Went instore today and it was £4.99!!! Presumably because Episode 2 is nearly upon us. Brilliant game if you don't have it in your collection, especially for this price.