A year and a half after Half-Life 2 first hit PCs, the highly acclaimed PC shooter is on the verge of getting its first expansion. Today, Valve Software announced that Half-Life 2: Episode One has gone gold and is on track to ship next Thursday, June 1. The add-on will cost $19.99 and will be sold both online via Valve's Steam download service and in boxed retail form, which Electronic Arts will distribute.

Like Half-Life 2, Episode One will be rated M for Mature due to sci-fi violence. However, unlike most expansions, it does not require the original game to play. That may be because the game goes outside City 17, the setting of much of Half-Life 2, to follow hero Gordon Freeman and supporting heroine Alyx, who plays a more prominent role this time around. Along with the cold announcement, Valve released a brand-new trailer showing Alyx in action.

In addition to two new multiplayer modes and the Lost Coast tech demo, Episode One will sport a preview of its sequel, Half-Life 2: Episode Two. The expansion, the existence of which was revealed in February, will add another four- to six-hour minicampaign to the Half-Life 2 saga when it is released later this year. Previously, the game had no official release window or date.

Today's gold announcement was the first official confirmation that a third Half-Life 2 episodic update is in development. Like Episode Two, Valve divulged little in the way of information about Episode Three, saying only that it was the last "in a trilogy...that will conclude by Christmas of 2007."