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.
Valve views episodes One through Three as tantamount to a standalone release; they are essentially Half-Life 3 divided into three episodes. Episode One is available as part of a bundle package known as The Orange Box, which also includes Half-Life 2, Episode Two, Team Fortress 2, and Portal. The game received a generally positive critical reaction, with the co-operative aspects of the gameplay singled out for particular praise.
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. 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. 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. 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.
While no new locales were introduced in Episode One, large alterations were made to the appearance of both City 17, where the game takes place, and the Citadel from the end of Half-Life 2 to reflect the changing shape of the world and to remind the player that their actions have had major effects on the storyline. The Citadel has degenerated from the cold, alien, and imposing fortress of the previous game to an extremely unstable state. This provides a visual cue to the player of the catastrophic damage they inflicted, and it allows for new gameplay elements that accentuate the dangers that come with the Citadel's imminent collapse. In addition, it serves a thematic purpose in highlighting the weakening of the Combine's dominance in City 17. Likewise, City 17 has been altered to reflect the aftermath of the resistance's open rebellion, with vast swathes of destroyed buildings, and the introduction of foes previously kept outside its confines in Half-Life 2 to further emphasize the scale of the uprising.
Half-Life 2 picks up the story with the G-Man taking Freeman out of stasis and inserting him on a train en-route to City 17 an indeterminate number of years after the Black Mesa Incident, with Earth now enslaved by an alien race known as the Combine. The player guides Gordon in aiding humanity's struggle against the Combine and their human representative, Dr. Wallace Breen. He oversees the occupation from his base of operations in the Citadel, a monolithic building at the heart of City 17. Fighting alongside Gordon is an underground resistance led by former colleague Dr. Eli Vance, as well other allies including Dr. Vance's daughter Alyx Vance and the enigmatic Vortigaunts, an alien species. Half-Life 2 ends with a climactic battle atop the Citadel that inflicts critical damage to its dark fusion reactor. When it seems as if Alyx and Gordon are to be engulfed by the explosion, the G-Man appears once more. After giving a cryptic speech, he extracts Gordon from danger and places him in stasis once again.
Episode One begins after the explosion of the reactor from which Gordon was extracted by the G-Man and where Alyx Vance was left behind. As time stops, several purple Vortigaunts appear and rescue Alyx from the blast. After she is rescued, the Vortigaunts appear before the G-Man and stand between him and Gordon. They teleport Gordon away from the scene, much to the G-Man's displeasure.
Gordon wakes up outside the Citadel, buried under a pile of rubble. He is uncovered and reunites with Alyx, who is relieved to see him. Alyx contacts Eli Vance and Isaac Kleiner, who have managed to escape the city, and is informed the Citadel's core is at risk of exploding at any moment. Kleiner states the explosion could be large enough to level the whole of City 17, and the only way for them to survive would be to re-enter the Citadel and slow the core's progression toward meltdown. Eli reluctantly agrees when he sees no other option.
Alyx and Gordon re-enter the now-decaying Citadel to try to stabilize the core. Along the way, they discover a Combine Advisor fleeing in a pod, but they are not aware of what it is at the time. Gordon and Alyx are successful in re-engaging the reactor's containment system, delaying the explosion, but Alyx discovers the Combine are deliberately accelerating the destruction of the Citadel to send a message to the Combine Overworld. She makes a copy of the message, causing the Combine to prioritize them as targets. In addition, Alyx locates and downloads a transmission from Dr. Judith Mossman describing a certain project she has located, before being cut off by a Combine attack. Alyx and Gordon then board a train to escape the Citadel.
The train derails en-route, forcing the duo to proceed on foot. As they fight through the disorganized Combine forces and rampant alien infestations, Kleiner appears on the screens Breen once used to pass out propaganda, and gives out useful updates to the evacuating citizens about the latest turn of events as well as reiterating the Citadel's imminent collapse. Alyx and Gordon eventually meet up with Barney Calhoun and a group of other survivors who are preparing to move on a train station to escape City 17. Alyx and Gordon provide cover for the passengers as they board.
To keep the survivors safe, Alyx and Gordon opt to take a different train. They manage to escape just as the reactor begins to detonate; the energy sends out the Combine's message. Several pods containing Combine Advisors are ejected from the Citadel as it detonates. The resulting shockwave catches the train, derailing it. The last thing the player hears is the twisting of metal and Alyx whispering Gordon's name. The fate of Alyx and Gordon is revealed in the sequel, Half-Life 2: Episode Two.
Lead writer Marc Laidlaw expanded further on the game's premise in the same article:
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.
It was later confirmed that players would reprise the role of Gordon Freeman— unlike the original Half-Life expansion packs, which all dealt with different characters. Valve explained their desire to develop Episode One in-house as opposed to working with outside contractors as with previous expansions as a decision based on their comfort with the technology and construction tools of Half-Life 2, as well as their enjoyment of working on Half-Life 2 content.
Originally called Half-Life 2: Aftermath, the expansion was renamed to Episode One as 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.
Valve made modifications to Alyx's AI that allowed her to react to the player's actions because of her significant involvement in the game. Modifications included commentating on objects the player manipulates or obstacles they have overcome. She also acts as an important device in both plot exposition and directing the player's journey, often vocalizing what the player is required to do next to progress. The developers explained that a large part of their focus was creating not only a believable companion for the player, but also one that did not obstruct the player's actions; allowing the player to dictate their own pace and method of overcoming any challenges faced without being hindered— this meant often scaling back Alyx's input and dialogue during the player's journey so they would not feel pressured to progress and consequently object to her presence. The developers also placed what they described as "hero moments" throughout the game that allow the player to single-handedly overcome certain obstacles, such as particularly challenging enemies, with Alyx taking the role of an observer and giving the player praise and adulation for their heroic feats. As is usual policy with Valve, play testers were used extensively by the developers throughout the entirety of the game's creation in order for Valve to continually gauge the effectiveness of in-game scenarios as well as the difficulty.
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. Upgrades to enemy AI allow Combine soldiers to utilize tactics previously unavailable to them, in particular the ability to crouch while being fired upon in order to "duck" underneath the player's line of fire. This feature is showcased to the utmost for the first time in the large street battle the player progresses through before entering the rebel safehouse in the game's fourth chapter. It also features the commentary node system debuted in the Lost Coast tech demo. The game's soundtrack was composed by Kelly Bailey. The music is used sparingly throughout, primarily playing during scenes of major plot developments or particularly important action sequences such as large battles or encountering a new enemy.
Episode One retailed at launch for $19.95 in the United States and is currently available over Valve's Steam platform for $9.95. The game is also being distributed through traditional retail channels by Electronic Arts both as a standalone and as part of the Half-Life 2: Platinum Collection. The game became available for pre-load and pre-purchase through Steam on May 1, 2006, with Half-Life Deathmatch: Source and Half-Life 2: Deathmatch immediately available for play as part of the package. Episode One is also included in the PC, Xbox 360 and Playstation 3 versions of The Orange Box.
Despite a primarily positive reception, a common criticism of the game has been its short length. Depending on the player, the game can take less time to complete than the company line of 4–6 hours, which has caused observers to raise the issue of whether it justifies the price tag. Other reviews have argued the futility of reviewing the game due to its episodic nature; as the first part of a three-part story arc it is difficult to judge it divorced from the final product. Reviewers also expressed disappointment at a lack of new features, including environments and weapons.