Half-Life 2 takes place approximately twenty years after the incident at the Black Mesa Research Facility from the first game, in which scientists accidentally opened a portal to the hostile dimension Xen. The game begins with Gordon Freeman being awoken from stasis by the mysterious G-Man, who reveals that the Black Mesa incident attracted the attention of a multidimensional empire called the Combine, which conquered Earth in seven hours. The Combine have implemented a brutal police state by biologically assimilating humans and other species. The G-Man inserts Gordon into a train arriving at City 17, the site of the Combine Citadel, where Dr. Wallace Breen, the former Black Mesa administrator who negotiated Earth's surrender governs as the Combine's puppet ruler.
