In this chapter, the HECU determine that they’re fighting a losing battle against the Xen aliens and, having continually failed to neutralize Freeman, abandon their mission and begin to evacuate the facility, calling in air strikes and heavy artillery to cover their retreat. Meanwhile, Gordon makes his way into an underground storage garage to avoid the bombardments, and eventually fights his way through scores of aliens and several groups of the decimated HECU forces before reaching the Lambda Complex.
The Marines begin to evacuate Black Mesa and air strikes begin. Gordon navigates underground water channels and tries to avoid scores of alien invaders as they pick off remaining Marine stragglers.

After arming himself with advanced prototype weaponry, Gordon again reaches the surface. It has become a war zone. The Vortigaunts, along with Alien Grunts and Gargantuas, have begun making determined attacks against the Marines, and the tide of the battle is beginning to turn in favor of the aliens.

The Marines call in reinforcements, but it isn’t enough. Gordon must scale cliffs and navigate the bombed out buildings while avoiding both sides as the forces of Xen begin to dominate the battlefield. Finally, Gordon reaches relative safety underground.

After escaping, Gordon eventually winds up in a secret part of the facility where he discovers that scientists had been “collecting” specimens from Xen long before this whole mess occurred. In it, Gordon Freeman continues to rearm himself against the HECU, and rejoins the battle against these enemies.

Gordon also encounters the alien military units, the Alien Grunts, and he finds one of the most powerful weapons in the Half-Life universe, the Tau Cannon, after a security guard accidentally blows up himself, a scientist, and a wall by overcharging it. Soon, he heads outside to search for the Lambda Complex.

After escaping from the trash compactor and getting back to the surface, Gordon stumbles towards a mostly abandoned part of the facility, meant for processing and disposing of waste and hazardous materials. Dodging and jumping through antiquated compaction machines, great vats of green waste, and slipping through furnaces, as well as riding on gravity-defying conveyor belts, Freeman escapes to a hidden part of the facility.