The original Half-Life takes place at a remote civilian and military laboratory called the Black Mesa Research Facility. During an experiment, researchers at Black Mesa accidentally cause a "resonance cascade" which rips open a portal to an alien world, Xen. Creatures from Xen flood into Black Mesa via the portal and start killing anyone in sight. The player takes on the role of Gordon Freeman, one of the research scientists involved in the accident, guiding him in his attempt to escape the facility. At the end of the game, Gordon is extracted by a mysterious figure colloquially known as the G-Man who "offered" him employment. Gordon is subsequently put into stasis by the G-Man.
Half-Life 2 picks up the story with the G-Man taking Freeman out of stasis and inserting him on a train en route to City 17 an indeterminate number of years after the Black Mesa Incident. Official sources differ on the actual length of this intermission—a story fragment written by author Marc Laidlaw for the development team puts the intermission at ten years, while Half-Life 2: Episode One's Web site puts this intermission as "nearly two decades" after the end of the events of Half-Life.