Gordon Freeman arrives in the borderworld, Xen.

On the strange borderworld, Gordon encounters many of the aliens that had been teleported into Black Mesa in their natural habitats, as well as the remains of HEV-wearing researchers that came before him.

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In this chapter, Gordon arrives at the Lambda Complex, where he believed the science team to be sheltering from the Xenians, only to find that the majority of the complex is now overrun by hostile creatures. He is informed that he must flood the reactor chambers in order to access the core, and that a team of scientists is waiting for him at the tip of the reactor.

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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.

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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.

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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.

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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.

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Continuing towards the surface, Gordon encounters a terrifying new aquatic creature, the Ichthyosaur, and acquires a deadly new weapon to fight it. After an underwater battle with the creature, he manages to escape and get back on his way.
After battling with the Black Ops, another “rescue” crew trying to kill even the marines, Freeman is captured by the HECU. They remove all of his weapons, and leave him for dead in a trash compactor. Thankfully, Freeman escapes once again, making his way back into the battle.

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As Gordon Freeman rides a monorail in the Materials Transport section of Black Mesa, he learns from a security guard that he must ride to the High Altitude Launch Center to launch a satellite, which the Lambda Team can use to reverse the effects of the Resonance Cascade.

He also discovers the HECU’s true hatred for him, as well as larger reinforcements: sandbag bunkers with rapid-fire sniper rifles, mounted machine gun turrets (manual and automated), and even mounted rocket launchers.

After going round in circles, it seems, for a while, he goes up several elevators and at last reaches the surface again. At the launch site, he sends the rocket up into orbit, and regretfully goes back below ground.

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Trying to reach the surface once more, Gordon’s progress is thwarted by a clash between a Gargantua and HECU soldiers in the Sector E Materials Transport.

He then learns from a dying security guard that if he restores the generator, he can ride the monorail to the surface. Freeman enters the Houndeye-infested subterranean areas and activates the generator, restoring power to the vicinity.

After activating more switches, he destroys the Gargantua patrolling the area by turning on the huge transformers supplying power to the tram line.
Hitching a ride, he smashes through a barricade of concrete blocks before continuing on his way.

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Gordon Freeman enters an abandoned, largely sealed-off area of Black Mesa dating back to its earlier days as a missile complex, full of mothballed machinery and now used as a dumping ground for toxic waste from the rest of the facility, where the HECU soldiers have yet to get a good foothold because of the high concentration of aliens in the area.

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