 A mysterious alien artifact has been recovered and brought to a top-secret research facility in the Black Mesa. When newcomer Dr. Gordon Freeman is assigned to physically test the artifact, things go very wrong when the artifact opens a destructive and unstable portal to Xen, a planet in a different dimension. Gordon blacks out, and awakes in a now crippled and ruined research facility. When Freeman and the survivors of this ordeal discover the portal has allowed its inhabitants-deadly alien creatures-to enter the facility, they realize that they must call the government for help. The government, however, already has plans to contain this incident by silencing the witnesses.