Half-Life 2: Episode One is the first of a trilogy of expansion packs/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. The player is 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.
Episode One's focus is on character development, in particular that of Gordon's female sidekick and friend Alyx Vance, to the extent that she accompanies the player for virtually the entire game: "It's kind of ironic that despite so much of the theme of Half-Life 2 being about other characters and other people, you spent most of the game alone," project lead Robin Walker said in the episode's announcement article in PC Gamer UK.
The announcement article also saw Marc Laidlaw explain the game's premise:
"Episode One deals with the events and issues set in motion during Half-Life 2. You've done critical damage to the Citadel. The whole place is going to go up, taking out City 17 and what's in its immediate radius. You and Alyx are leading the flight from the city getting up close and personal with some of the creatures and sights from the end of the game."
Despite this comment and much fan speculation, the Combine Crab and Mortar synths were not present in Episode One; Stalkers and previously glimpsed areas of the Citadel are instead encountered and explored by the player.
After some initial confusion, sparked partly with an attempt at humor by PC Gamer UK, which suggested that Alyx was Episode One's playable character, it was confirmed that players would indeed play as Gordon Freeman – unlike the original Half-Life expansion packs, which all dealt with different characters. Part of the reason for this change of direction may lie with the in-house development of Half-Life 2: Episode One: previously, Half-Life expansions were developed by third party Gearbox Software (albeit with scripts produced by Valve).
The game runs on an incrementally upgraded version of Valve's proprietary Source engine, and features both the engine's high dynamic range rendering capabilities, and a new version of its facial animation/expression technology. It also features the commentary node system debuted in the Lost Coast tech demo.
The renaming of Aftermath to Episode One was an indication of Valve's confidence with their episodic structure, an implication confirmed in February and May of 2006, with news of a trilogy of episodes covering the present story arc. While the plots and dialogue of Half-Life and Half-Life 2 were written solely by Valve's in-house writer Marc Laidlaw, the "Half-Life 2 Episodes" are collaboratively written by Laidlaw, Chet Faliszek and Erik Wolpaw with Laidlaw retaining overall leadership of the group