Barney Calhoun and Rosenberg reach an older part of the Black Mesa Research Facility where a disused teleporter system is being reassembled by scientists.

To make the teleporter completely operational, Calhoun is forced to travel to Xen and activate a device to allow teleportation to happen. Notable in the older part of the facility are older-looking models of health and HEV chargers and the use of hand scanners instead of retina scanners for identification.

Barney Calhoun reaches the surface and encounters the HECU. He manages to fight through them and reach Dr. Rosenberg. He and a team of fellow scientists were trying to escape until the marines caught them.

Calhoun rescues some scientists who have been locked in railway cars in Black Mesa’s classification yards. One of the scientists, Dr. Rosenberg (who is also a character in Decay), advises Calhoun that the entire facility is surrounded by Hazardous Environment Combat Unit Marines intent upon killing all base personnel to cover-up Black Mesa’s research into extradimensional exploration.

The only way to escape, Rosenberg explains, is to use an old prototype of the Lambda Complex teleporter; if it works, they can reach an obscure entrance to Black Mesa which (hopefully) has been overlooked by the Marines.

Barney Calhoun regains consciousness at the bottom of the elevator shaft to the sight of a houndeye eating the body of a fellow security guard.

The two scientists in the elevator are dead, and so Calhoun sets off through the industrial waste areas of Black Mesa to try to find help.

Along the way he gains an insight into the scope of the disaster, and sees a pair of marines dumping corpses into a sewer opening, thus discovering that the military is trying to cover up the disaster rather than evacuate surviving personnel.

As a seemingly normal day at work starts, Barney Calhoun visits security headquarters, including a shooting range and a surveillance room where it is possible to see Gordon Freeman again on camera heading towards the HEV Storage Area.

Calhoun is then sent to help some scientists with an elevator; along the way, he witnesses many incidents foreshadowing the resonance cascade, such as a pair of scientists vainly trying to fix a supercomputer. He also sees the G-Man passing by in a tram.