[6] Episode One integrates tutorial-like tasks into the story to familiarize the player with new gameplay mechanics without breaking immersion.

[7] The artificial intelligence (AI) for Alyx Vance, Gordon's companion, was explicitly designed for co-operative play in Episode One to complement the player's abilities.

[11] Similarly, Alyx will often take up strategic positions and provide covering fire to keep the player safe while they travel to a certain area or perform certain actions.

Alyx makes contact with Dr. Eli Vance (Robert Guillaume) and Dr. Isaac Kleiner (Harry S. Robins), who have escaped the city.

She downloads a copy of the transmission and a video recorded by Judith Mossman (Michelle Forbes), in which she discusses the location of an unknown project before being subdued by a Combine attack.

They proceed underground and through the city streets, fighting past disorganized Combine forces and rampant Xen wildlife.

Near a Combine-held train station, Gordon and Alyx reunite with Barney Calhoun (Michael Shapiro), to evacuate refugees from the city.

[14] The designer Robin Walker said the team had become comfortable with their tools, and wanted to capitalize on their experience instead of developing new technologies.

[15] Valve's president, Gabe Newell, said customers would be happier with a new Half-Life game delivered in a shorter time rather than waiting years for another "monolithic product".

[18] The music is used sparingly; it plays primarily during scenes of major plot developments or particularly important action sequences such as large battles or when encountering a new enemy.

[21] Extensive alterations were made to the appearance of both City 17 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.

[22] 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.

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.

[34] PC Gamer commented that "while this inaugural episode may not be the essential FPS that Half-Life 2 is, I can't imagine any shooter fan who'd want to miss it.

[7] Edge praised the "deftness" with which the game directed the player's eyes, and the strength of Alyx as a companion, concluding: "In an interactive genre bound to the traditions of the pop-up gun and invisible hero, it simply doesn't get more sophisticated than this.

[39] GameSpy ranked Episode One ninth on its 2006 "Games of the Year" list, and it also noted the implementation of Alyx as a believable and useful companion.