Half-Life 2: Episode One: "Half-Life 2: Episode One is the first in a series of new adventures created by Valve that extend the Half-Life 2 single player experience. It details the aftermath of Half Life 2 and launches a journey beyond City 17.

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. Battle side-by-side with her through Valve's first episodic game, a four-to-six hour adventure of greater density and detail than non-episodic releases.

In addition to the new single player campaign, Half-Life 2: Episode One also includes two multiplayer games. And everything you need to play is included in the box � Half-Life 2 is not required to play.
Features
An all-new episode for one of the best-selling action games of all time.
Discover what�s become of City 17, Dr. Breen, and the G-Man.
Incredibly advanced Artificial Intelligence (AI).
Highly detailed environments, featuring High Dynamic Range lighting, provide amazing gameplay settings.
Digital Actors: The most sophisticated in-game characters ever witnessed.
Physical Gameplay: Objects obey the laws of gravity, friction, and buoyancy.
Design Team Commentary Mode.
Also includes two multiplayer games: Half-Life 2 Deathmatch and Half-Life 1 Deathmatch: SourceTM.
Half-Life 2 not required to play.
About Half-Life 2
Half-Life 2 has been named Game of the Year by over 35 organizations and sold over four million copies worldwide. The intense, real-time gameplay delivered in Half-Life 2 and Half-Life 2: Episode One is made possible "