Theoretical physicist Gordon Freeman arrives late for work at the Black Mesa Research Facility. As part of an experiment, he pushes an unusual material into a machine, called the anti-mass spectrometer, for analysis. The spectrometer explodes, creating a "resonance cascade" that severely damages the facility and opens a portal to another dimension, Xen. Surviving scientists urge Gordon to head to the surface, where he defends himself against hostile Xen creatures and the HECU (Hazardous Environment Combat Unit), a special unit of United States Marines sent to cover up the incident by killing all of the hostile aliens and any Black Mesa personnel they find.
