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== Plot ==
At the underground [[Black Mesa Research Facility]], physicist [[Gordon Freeman]] participates in an experiment on a crystal of unknown origin. This triggers an explosion which severely damages the facility, and inexplicably causes alien creatures to appear. Many scientists are killed.
Theoretical physicist [[Gordon Freeman]] arrives late for work at the [[Black Mesa Research Facility]]. As part of a scientific experiment, he pushes an unusual crystal of alien origin into a machine known as the anti-mass spectrometer. The spectrometer explodes, creating a "[[positive feedback|resonance cascade]]" that severely damages the facility and opens a portal to an alien dimension, Xen. The surviving scientists urge Gordon to travel to the surface, where he defends himself against hostile Xen creatures and the Hazardous Environmental Combat Unit (HECU), a [[special forces]] unit of the [[United States Marine Corps]] dispatched to eliminate the Xen creatures as well as all surviving Black Mesa personnel to [[cover-up|cover up]] the incident.
Gordon survives thanks to his hazard suit. Venturing to the surface for help, he discovers that hostile [[United States Marine Corps|Marines]] have been dispatched to [[cover-up|cover up]] the incident. Escaping below ground, he destroys a giant tentacled creature inside a [[rocket engine test facility]].
Gordon learns that scientists from the Lambda Complex may have found a way to close the portal. Gordon travels to the other end of the facility to assist them. Along the way, he activates a [[rocket engine test facility]] to destroy a giant tentacled creature and uses a disused railway system to reach and launch a satellite rocket, with the purpose of attempting to close the portal via satellite, which ultimately fails. Freeman is captured by HECU marines and left to be killed in a garbage compactor, but escapes and makes his way to an older part of the facility. There, he discovers Xen specimens collected before the incident.
Freeman travels by underground rail to the Lambda Complex, where scientists can stop the alien invasion. Along the way, he launches a space satellite to aid the scientists.
Overwhelmed by the alien forces, the HECU pulls out of Black Mesa and begin [[airstrike]]s. Scaling cliffs, navigating destroyed buildings, and traversing through underground water channels, Gordon arrives at the Lambda Complex, where scientists learn the portal is being forced open on the other side by a large entity. They have developed teleportation technology that allows Gordon to travel to Xen, where he is tasked with stopping the entity.
He is eventually captured by Marines and left for dead in a garbage compactor. He escapes through a waste treatment facility, and stumbles into a lab filled with alien specimens, seemingly collected before the accident.
In Xen, Gordon encounters the remains of researchers who ventured there before him, and Gordon defeats the Gonarch, a large egg-laying headcrab. At an alien factory manufacturing alien soldiers, he enters a portal that sends him to a vast cave. There, Gordon confronts the Nihilanth, the entity maintaining the rift, and destroys it. Gordon is summoned by the mysterious [[G-Man (Half-Life)|G-Man]], who has been watching his progress in Black Mesa. The G-Man commends him and explains that his "employers" wish to employ Gordon. If Gordon refuses, he is teleported to an area full of alien soldiers to be killed immediately. If Gordon accepts, the G-Man congratulates him and places him into stasis to await his next assignment.
The Marines are overwhelmed and withdraw. Scaling cliffs, navigating a bombed-out military base, and traversing sewers, Gordon arrives at the Lambda Complex. Inside, he restarts a nuclear reactor and uses mysterious teleportation technology to reach the last survivors of the science team.
They reveal that humans have been teleporting surveyors to an alien "border world". However, a hostile psychic entity opened an enormous portal back to Earth, triggering the invasion. The scientists send Gordon to kill the creature.
On the alien world Xen, Freeman navigates various floating islands, encountering the remains of researchers who ventured before him. He defeats the monstrous Gonarch, and explores a factory manufacturing alien soldiers.
Finally, Gordon kills the Nihilanth, the entity maintaining the rift. He is then disarmed and summoned by the powerful and mysterious [[G-Man (Half-Life)|Administrator]], who offers work for his "employers". If he accepts, he is placed into stasis to await his next assignment.
== Development ==
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