Half-Life 2: Episode One continues the story of Half-Life 2. It begins immediately after the end of this game, and follows Gordon Freeman (the player) and Alyx as they attempt to escape City 17 in the wake of the defeat of the Combine forces and the battle in the Citadel.
Gameplay and Gordon's weapon arsenal are identical to Half-Life 2. Enemies too are familiar, with only one new zombie variant. A notable gameplay feature is the behavior of Alyx, who fights along Gordon's side for virtually the entire game. While the player still provides most of the offensive firepower, Alyx does provide a beneficial presence, and is far from the typical cannon fodder AI.
The game has four to six hours of gameplay, and also features a "Commentary" mode, which enables hotspots on each map where the player can listen to developer commentary explaining how or why certain things were done.