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 have to traverse linear stages, disposing of enemies and solving physics puzzles. This time Gordon will be accompanied and assisted by Alyx. Her AI will attempt to adjust to the player choices. While purchasing Episode One, players will receive a copy of Half-Life: Deathmatch with it. Game mechanics and puzzles will center around Gravity Gun, which will be the first weapon in players possession from the beginning.