"Half-Life 2: Episode One is the first in a trilogy of episodes that serve as the sequel for the 2004 first-person shooter video game Half-Life 2.
The game's events take place immediately after those in Half-Life 2, in and around war-torn City 17. Episode One follows scientist Gordon Freeman and his companion Alyx Vance as they fight in humanity's continuing struggle against the transhuman race known as the Combine. When the story begins, Gordon wakes up outside the enemy's base of operations, the Citadel, after being left unconscious from the concluding events of Half-Life 2. During the course of the game, Gordon travels with Alyx as they attempt to evacuate the city. As the game comes to an end, Gordon and Alyx are caught in a major accident, and both of their fates are revealed in the sequel, Episode Two."
Source: Wikipedia, "Half-Life_2:_Episode_One," available under the CC-BY-SA License.