Direct continuation of the Half-Life 2, Episode One serves as a direct sequel to the story. After the mystically assisted escape from the citadel, Gordon and Alyx are reunited outside, but the Citadel core became unstable and will imminently self-destruct. And the only way to save citizens of City 17 is to re-enter the decaying building and slow down the destruction for long enough, to evacuate the civilians and families. Continuation of the story uses the same Source engine, and players will h...