The game is somewhere between a sequel and an expansion pack, picking up the story of Adrian Shephard, the US Marine who's the focus of Half-Life: Opposing Force (the expansion pack for the original Half-Life). It's set in the Nova Prospekt prison during the events of Half-Life 2, with Gordon Freeman cornered in the alien stronghold and Shephard teleported in by his allies, the Vortigaunts, to help out. Needless to say, while Prospekt will function as a stand-alone title (you won't need to own Half-Life 2 to play it), it's very much aimed at established fans of the series.