This article describes the Half-Life storyline as it is played in-game, chapter by chapter.
 Hazard Course (optional)
This chapter is the training segment of Half-Life and takes place in the Black Mesa Hazard Course, a Training Facility. It teaches the player how to use the HEV Suit, how to move around and interact with the environment, how to use weapons, and how to use the Long Jump Module. Scientists observe Gordon Freeman's progress as he moves through the course.
 Chapter 1: Black Mesa Inbound
This is the first chapter of Half-Life. It serves as the introduction to the game and follows the player character, Gordon Freeman, as he heads to Sector C. Gordon rides a tram deep into the heart of the Black Mesa Research Facility on his way to the Anomalous Materials Lab to begin work. Nothing seems to be out of place, except a chemical leak that forces the tram to momentarily stop. A man in a dark suit is seen watching Gordon from another tram, later to be known as the G-Man.
 Chapter 2: Anomalous Materials
Later on, once he reaches his destination, Gordon glimpses the G-Man in an argument with a scientist behind soundproof glass. After donning his HEV Suit, he proceeds to the Test Chamber, containing a large Anti-Mass Spectrometer. Immediately after pushing an alien sample into the scanning beam, the system goes critical, triggering a Resonance Cascade. This opens an inter-dimensional portal between Earth and a bizarre alien world known as Xen. Gordon is briefly teleported there during the ordeal, at one point encountering what appear to be sentient life forms, including Vortigaunts. Gordon blacks out shortly after.
 Chapter 3: Unforeseen Consequences
This chapter details Gordon's efforts to reach the surface to get help in the aftermath of the Resonance Cascade. This chapter introduces the player to weapons, combat, and environmental hazards, among other things.
Waking up back in Black Mesa, Gordon makes his way out of the Test Chamber. He manages to reach his colleague, Eli Vance, who informs him that all communication outside Black Mesa has been lost. Eli requests that Gordon try to reach the surface in order to find help. Black Mesa has suffered massive structural damage, and aliens from Xen begin randomly teleporting in all around the facility. Most are simply reacting naturally to their new surroundings, but even more hostile are the creatures glimpsed earlier, known as Vortigaunts, who seem to attack with a single-minded intent to kill. Gordon also occasionally witnesses the G-Man, watching him from out-of-reach places.
Since the tram system is too damaged to function, Gordon must cross Sector B's Coolant Reserve Facility in order to find a way out.
 Chapter 4: Office Complex
Gordon makes his way to the office complex where several scientists have barricaded themselves in their rooms, hiding from the rampant aliens overrunning the complex. Gordon overhears rumors from these scientists that the military has arrived to rescue the people trapped in the facility. As the elevators are not working, he climbs an elevator shaft to proceed on his journey to the surface.
 Chapter 5: "We've Got Hostiles"
This chapter finds Gordon passing through various storage and staging areas, continuing his hunt for a way to the surface. He learns that the soldiers rumored to be coming on a rescue mission have already arrived. While attempting to join up with them, Gordon witnesses a soldier killing an innocent scientist, revealing to him that the Hazardous Environment Combat Unit is there not only to eliminate the alien threat but also everyone else connected to the project.
Gordon continues battling through hostile forces, both alien and human, until eventually reaching the surface. He finds the area completely under the military control and in the middle of a mortar bombardment, forcing him to retreat back into the complex. Back underground, Gordon learns from a scientist that the surviving members of the Lambda Team, set up in the Lambda Complex at the other end of the facility, may be able to close the dimensional rift. Gordon starts a trek across Black Mesa to reach them.
 Chapter 6: Blast Pit
Gordon enters an abandoned, largely sealed-off area of Black Mesa, dating back to its earlier days during the Cold War as a missile complex. The area is full of mothballed machinery and is now used as a dumping ground for toxic waste from the rest of the facility. The HECU soldiers lack a foothold in this location due to the high concentration of aliens present. Here, Gordon encounters a deadly new foe, the Tentacles. After reactivating the fuel, power, and oxygen lines, Gordon manages to incinerate the Tentacles by activating the rocket engine at the top of the silo that the creatures were inhabiting. He leaves through the dirt tunnel that the Tentacles had receded into.
 Chapter 7: Power Up
Trying to reach the surface once more, Gordon's progress is thwarted by a clash between a Gargantua and HECU soldiers in Sector E Materials Transport. He learns from a dying security guard that, if he restores the power, he can ride the monorail to the surface. Gordon 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 luring it into the transformers supplying power to the tram line. Boarding a rail cart, Gordon smashes through a barricade of concrete blocks to proceed on his way.
 Chapter 8: On A Rail
As Gordon rides the monorail through the Materials Transport section of Black Mesa, he learns from a security guard that he must head to the High Altitude Launch Center to launch a satellite which the Lambda Team can use to reverse the effects of the Resonance Cascade. Gordon encounters further resistance from HECU forces, now actively hunting him in revenge for the large number of casualties he has carried out. He eventually makes his way back up to the surface through use of several elevators and makes his way to the launch site. There, he succeeds in sending the rocket into orbit but is once again forced to travel back underground.
 Chapter 9: Apprehension
Searching for a new way to the surface, Gordon encounters a deadly aquatic enemy, the Ichthyosaur, and acquires a crossbow to combat it. After an underwater battle with the creature, he manages to escape and continue on his way.
After battling Black Ops assassins, another faction sent in to clear the facility, Gordon is ambushed and captured by a group of soldiers. They seize his weapons and leave him unconscious in a trash compactor.
 Chapter 10: Residue Processing
Gordon wakes up and manages to escape from the trash compactor. He stumbles across a mostly abandoned portion of the facility meant for processing and disposing of waste and hazardous materials. Making his way through antiquated compaction machines, great vats of waste, and furnaces, Gordon eventually breaks into a hidden part of the facility.
 Chapter 11: Questionable Ethics
Gordon rearms himself against the HECU as well as various alien enemies running amok in the lab. He encounters for the first time the alien military units, the Alien Grunts, and manages to scavenge a powerful experimental weapon, the Tau Cannon, from the remains of a scientist and security guard. He presses on his search for the Lambda Complex.
 Chapter 12: Surface Tension
After arming himself with advanced prototype weaponry, Gordon again reaches the surface which has become a war zone. Aliens have begun making determined strikes against the soldiers, turning the tide of the battle in their favor. Gordon scales a cliff wall and navigates several bombed out buildings, combating both sides as the forces of Xen begin to dominate the battlefield. Finally, Gordon reaches relative safety underground.
- The original name of this chapter, Cliff Hanger, was used at E3 1998.
- Like "Half-Life", "Blue Shift", "Opposing Force", and "Decay", "Surface Tension" is a scientific pun, referring to the properties of water which makes certain objects float above it.
 Chapter 13: "Forget About Freeman!"
The HECU, after having taken heavy losses in their battle against the aliens and failing to neutralize Gordon, abandon their mission and begin to evacuate the facility, calling in air strikes and heavy artillery to cover their retreat. Gordon makes his way into an underground storage garage to avoid the bombardments and navigates his way through underground water channels, fighting his way through scores of aliens and the remaining HECU forces before finally reaching the Lambda Complex.
 Chapter 14: Lambda Core
Gordon arrives at the Lambda Complex, but instead of the science team, he finds 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. Gordon navigates through the Lambda Core, facing numerous assaults from the Xen forces, now the primary enemy after the evacuation of the HECU.
After much struggle, Gordon finally reaches what is left of the science team where he discovers that the Lambda Complex is where the teleportation technology that allowed scientists to travel to Xen was developed. The handful of surviving personnel, who have held themselves up in a small stronghold, inform Gordon that the satellite he launched was not able to reverse the effects of the Resonance Cascade because an immensely powerful being on the other side was keeping the portal between the worlds open. Gordon learns that he must kill it to prevent the Xen aliens from completely taking over. To prepare for what lies ahead, Gordon arms himself and collects the Long Jump Module. After an attempted interception by Alien Controllers, the scientists activate the teleport and he is teleported to Xen.
The original chapter name was Down and Out.
 Chapter 15: Xen
Gordon arrives in the borderworld, Xen. In the strange alien dimension, he encounters many of the aliens that had been teleported into Black Mesa in their natural habitats as well as the remains of several HEV-clad researchers that had come before him. Gordon makes his way across the borderworld in search of the creature holding the portal open.
 Chapter 16: Gonarch's Lair
As Gordon makes his way through Xen, he is forced to fight a large alien creature, the Gonarch, in her lair. After killing it, Gordon continues his search.
It is possible that Gonarch's Lair was originally placed after Interloper as the Gonarch's Lair map names begin from c4a2, while Interloper map names begin from c4a1a.
 Chapter 17: Interloper
 Chapter 18: Nihilanth
Gordon is teleported into a vast cave in which he finally confronts the powerful and mysterious being holding the portal open, the Nihilanth. In the ensuing battle, Gordon destroys the crystal that allowed the Nihilanth to regenerate its strength, allowing him to attack its vulnerable brain. As the massive being enters its death throes, it floats toward the ceiling and explodes in a giant green blast that overpowers Gordon's senses.
When Gordon comes to, he is confronted by the G-Man. Both are transported to various locales around Xen while the G-Man praises Gordon's actions there. He explains that his "employers" are now in control of the borderworld for the time being and have authorized him to offer Gordon a job, agreeing that he has "limitless potential". The final teleportation places them inside a tram identical to the one Gordon rode on at the beginning of the game that is flying through space at immense speed. Gordon accepts this offer by stepping though a portal and hears the G-Man's voice one last time: "Wisely done, Mr. Freeman, I will see you up ahead." The story continues from this point in Half-Life 2.
Alternatively, the player may choose not to step into the portal in the final scene. This causes Gordon to be teleported into a giant cave facing rows of Alien Grunts and Alien Crafts as the game ends with the G-Man's comment, "Well, it looks like we won't be working together. No regrets, Mr. Freeman."
- The chapter name "Endgame" is given in the
titles.txtfile and in Half-Life's save and load dialogs.
- Half-Life series writer Marc Laidlaw has stated that the choice made at the end of the game was ultimately arbitrary as there was always to be one inevitable outcome, establishing the recurring theme of "free choice being an illusion" that later installments would follow.