Valve today said that Half-Life 2 : Episode One, the first game in a trilogy of episodes that will conclude by Christmas of 2007, has gone gold today. The developer said that this series will lead "far beyond City 17" in the original Half-Life 2, a game that Valve claims has sold 15 million times so far.

Half-Life 2 : Episode One does not require Half-Life 2 to play, and will be available via the firm’s Steam service and at retail stores in North America for just $20 on 1 June.

"Given the rapid changes in PC gaming, we expect episodic games will have the same impact on single-player gaming that Counter-Strike had on multiplayer," said Gabe Newell, president and co-founder of Valve. "We decided that a follow-up to Half-Life 2 delivered in 18 months with our first episode was more in tune with what our customers want than waiting six years for another monolithic product."

According to Valve, Episode One is designed to be played by an individual player and will provide gameplay for four to six hours. Stepping into the hazard suit of Dr. Gordon Freeman, the players "face the immediate repercussions of your actions in City 17 and the Citadel," the developer said. "Alyx Vance and her robot, Dog, will accompany you in your efforts to aid in the human resistance’s desperate battle against the totalitarian alien menace of the Combine."