Half-Life 2: Episode One is the first of a trilogy of episodes for the 2004 first-person shooter game, Half-Life 2.[1] It was developed by Valve Corporation and was released on June 1, 2006. The episode is a standalone game; while a continuation of Half-Life 2, it does not require the original game to be installed or registered to a user's online Steam account to play. It takes advantage of several major upgrades to the Source game engine since the release of Half-Life 2.
The episode takes place immediately after Half-Life 2 in and around the war-torn city of City 17. Episode One follows scientist Gordon Freeman and his companion Alyx Vance as they deal with the events of Half-Life 2 and humanity's continuing struggle against the Combine, an alien race.
There are no significant differences in gameplay between Half-Life 2 and Episode One.[5] Players make their way through a linear series of levels and encounter various enemies and allies. The gameplay is broken up between combat-oriented challenges and physics-based puzzles.[5] Episode One also integrates tutorial-like tasks into the story to familiarize the player with new gameplay mechanics without breaking immersion. The gravity gun continues to play a crucial function throughout the game as it has in the series, granting the player extensive use of physics in combat and puzzle-solving.[6]
The artificial intelligence (AI) for Alyx Vance was specifically designed for co-operative play in Episode One so she would complement the player's abilities. The developers described Alyx's programming for Episode One as a "personality code" as opposed to an "AI code", emphasizing their attention to creating a unique and believable companion.[7] In addition, she was specially programmed to avoid performing too many mechanical or repetitious actions, such as repeating lines of dialogue or performing certain routines in combat situations.[7] An example of this co-operative gameplay is combat in the darker underground levels just after the player escapes the Citadel, the Combine's base of operations. Here, the player can use their flashlight to help Alyx spot and kill oncoming enemies, conserving the player's ammunition.[8] Similarly, Alyx will often take up stationary gunner points to provide covering fire while the player travels to a certain area or performs certain actions to keep them safe while they undertake their tasks.[9]

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