Half-Life 2: Episode One is the first in a series of new adventures created by Valve that extends the Half-Life 2 single player experience.

Stepping into the hazard suit of Dr. Gordon Freeman, you face the immediate repercussions of your actions in City 17 and the Citadel. Rejoin Alyx Vance and her robot, Dog, to once again aid the human resistance in their desperate battle against the totalitarian alien menace of the Combine.

Episode One exposes Alyx's combat skills and knowledge of City 17 as the player battles side-by-side with her in this 4-6 hour adventure of greater density and detail than non-episodic releases.

The intense gameplay delivered in Episode One is made possible only by Source, Valve's proprietary engine technology, which has been enhanced with new rendering technology and advanced artificial intelligence to deliver new levels of graphics and character interaction.

Plot 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. Time suddenly freezes, and 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, and reunites with Alyx, who is relieved to see him. Alyx contacts Eli Vance and Isaac Kleiner, who have escaped 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 is 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 and are successful in re-engaging the reactor's containment system, which delays the explosion. Alyx discovers the Combine are deliberately accelerating the destruction of the Citadel to send a message to the Combine's homeworld. She makes a copy of the message, which causes the Combine to prioritize them as targets. Alyx then locates and downloads a transmission from Dr. Judith Mossman describing a certain project she has located, before being cut off by a Combine attack. Afterwords, Alyx and Gordon 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 are ejected from the Citadel as it detonates. The resulting shock wave 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.



Development
Half-Life 2: Episode One is the first in a trilogy of episodes serving as the sequel for the 2004 first-person shooter video game Half-Life 2. In February 2006, Valve announced that they would be releasing a trilogy of episodes covering the same 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" were collaboratively written by Laidlaw, Chet Faliszek, and Erik Wolpaw, with Laidlaw retaining overall leadership of the group.

Valve explained that the focus of Episode One was character development, in particular that of Gordon's female sidekick and friend Alyx, because she accompanies the player for virtually the entire game. Project lead Robin Walker discussed the reasoning behind this approach in an article announcing the game in the May 2005 issue of PC Gamer UK, saying, "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." Lead writer Marc Laidlaw expanded further on the game's premise, saying,

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 decided to develop Episode One in-house, as opposed to working with outside contractors as with previous expansions, because they were already comfortable with the technology and construction tools of Half-Life 2, and they also enjoyed working on Half-Life 2 content.

Because of Alyx's significant involvement in the game, Valve made modifications to her AI that allowed her to react to the player's actions. Modifications include 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. They wanted to allow the player to dictate their own pace and method of overcoming any challenges faced without being hindered. This meant that Valve often had to scale 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, which allow the player to single-handedly overcome obstacles such as particularly challenging enemies, during which Alyx takes the role of an observer and gives the player praise and adulation for their heroic feats. 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 upgraded version of Valve's proprietary Source engine, and features both the engine's advanced lighting effects, and a new version of its facial animation/expression technology. Upgrades to enemy AI allow Combine soldiers to utilize tactics previously unavailable to them. For example, Combine soldiers were given the ability to crouch while being fired upon in order to duck underneath the player's line of fire. The game's soundtrack was composed by Kelly Bailey. The music is used sparingly throughout; it plays primarily during scenes of major plot developments or particularly important action sequences such as large battles or when encountering a new enemy.

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 remind the player that their actions have major effects on the storyline. The Citadel has degenerated from the cold, alien, and imposing fortress of the previous game into an extremely unstable state. This provides a visual cue to the player of the catastrophic damage they inflicted, and it allows for the introduction of new gameplay elements that accentuate the dangers which come with the Citadel's imminent collapse. In addition, it serves a thematic purpose by 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 emphasize the scale of the uprising.