The Half-Life series begins in the early 21st century, at the fictional Black Mesa Research Facility in New Mexico. Gordon Freeman, a recently employed theoretical physicist, is involved in an experiment analyzing an unknown crystalline artifact; however, when the anti-mass spectrometer beam contacts the crystal, it opens a dimensional rift between Black Mesa and another world called Xen, causing monsters to swarm Black Mesa and kill many of the facility’s personnel. Attempts by the Black Mesa personnel to close the rift are unsuccessful, leading to a Marine Recon unit being sent in to silence the facility, including any survivors from the science team. Freeman fights through the facility to meet with several other scientists, who decide to travel to the alien dimension to stop the aliens. On Xen, Freeman eliminates the alien “leader” and is confronted by the G-Man, who offers Freeman employment before putting him into stasis. Back in Black Mesa, a second alien race begins an invasion, but is stopped when a Marine corporal, Adrian Shephard, collapses its portal in the facility. The G-Man then destroys Black Mesa with a nuclear warhead, and detains Shephard in stasis.
Nearly twenty years later, the G-Man brings Freeman out of stasis and inserts him into a dystopian Earth ruled by the Combine, a faction consisting of human and alien members, that used the dimensional rift caused at Black Mesa to conquer Earth in the interim. In the Eastern European settlement City 17, Freeman meets surviving members of the Black Mesa incident, including Isaac Kleiner, Barney Calhoun, Eli Vance and his daughter Alyx Vance, and aids in the human resistance against Combine rule. The Xen aliens, the Vortigaunts, who have been enslaved by the Combine, also assist the resistance. When his presence is made known to former Black Mesa administrator and Combine spokesman Wallace Breen, Freeman becomes a prime target for the Combine forces. Eventually, Freeman sparks a full revolution amongst the human citizens. Eli Vance and his daughter are subsequently captured by the Combine, and Freeman helps the resistance forces attack the Combine’s Citadel to rescue them. Breen attempts to flee in a teleporter, but is presumed dead after Freeman destroys the dark energy reactor at the Citadel’s top.
The G-Man then arrives to extract Freeman before he is engulfed in the explosion, but is interrupted when Vortigaunts liberate Freeman from stasis and place both him and Alyx Vance at the bottom of the Citadel. The two then attempt to stabilize the Combine’s primary reactor while the citizens evacuate the city, discovering that the Combine are attempting to destroy the reactor to call for reinforcements from the Combine’s native dimension. After downloading critical data, they move through the war-torn city to the train station to take the last train out of the city. The Combine then destroy the reactor and thus both the Citadel and the city; the resulting explosion causes the train to derail.
Freeman awakens in one of the wrecked train cars with Alyx outside, where a forming superportal is visible from where the Citadel used to stand. They begin a journey through the White Forest to a resistance-controlled missile base in the nearby mountains. Along the way, Freeman and Alyx are ambushed and Alyx is severely injured. However, a group of Vortigaunts are able to heal her. During the ritual to heal Alyx, Freeman is briefly spoken to by the G-Man in a hallucination-like vision, indicating that he has caught up to Freeman again. They are able to reach the resistance base and deliver the data, which contains the codes to destroy the portal as well as information on the Borealis, an enigmatic research vessel operated by Black Mesa’s rival, Aperture Science. The base then launches a satellite that is able to shut down the superportal, cutting off the Combine from outside assistance. However, as Alyx and Freeman prepare to travel to the Arctic and investigate the Borealis, they are attacked by Combine Advisors, who kill Eli Vance before being driven off by Alyx’s pet robot, Dog.
Portal’s plot is revealed to the player via audio messages from GLaDOS and side rooms found in the later levels. According to The Final Hours of Portal 2, the year is established to be “somewhere in 2010” — twelve years after its abandonment. The game begins with protagonist Chell waking up from a stasis bed and hearing instructions and warnings from GLaDOS about the upcoming test experience. This part of the game involves distinct test chambers that, in sequence, introduce players to the game’s mechanics. GLaDOS’s announcements serve not only to instruct Chell and help her progress through the game, but also to create atmosphere and develop the AI as a character. Chell is promised cake and grief counseling as her reward if she manages to complete all the test chambers.
Chell proceeds through the empty Enrichment Center, interacting only with GLaDOS. Over the course of the game, GLaDOS’s motives are hinted to be more sinister than her helpful demeanor suggests. Although she is designed to appear helpful and encouraging, GLaDOS’s actions and speech suggest insincerity and callous disregard for the safety and well-being of the test subjects. The test chambers become increasingly dangerous as Chell proceeds, and GLaDOS even directs Chell through a live-fire course designed for military androids as a result of “mandatory scheduled maintenance” in the regular test chamber. In another chamber, GLaDOS boasts about the fidelity and importance of the Weighted Companion Cube, a waist-high crate with a single large pink heart on each face, for helping Chell to complete the chamber. However, GLaDOS then declares that it “unfortunately must be euthanized” in an “emergency intelligence incinerator” before Chell can continue. Some of the later chambers include automated turrets with child-like voices (also voiced by McLain) that fire at Chell, only to sympathize with her after being disabled (“I don’t blame you” and “No hard feelings”).
After Chell completes the final test chamber, GLaDOS congratulates her and prepares her “victory candescence”, maneuvering Chell into a pit of fire. As GLaDOS assures her that “all Aperture technologies remain safely operational up to 4,000 degrees [sic] kelvin”, Chell escapes with the use of the portal gun and makes her way through the maintenance areas within the Enrichment Center. GLaDOS becomes panicked and insists that she was only pretending to kill Chell, as part of testing. GLaDOS then asks Chell to assume the “party escort submission position”, lying face-first on the ground, so that a “party escort robot” can take her to her reward. Chell, thinking that GLaDOS is trying to trick her into a vulnerable position, continues forward. Throughout this section, GLaDOS still sends messages to Chell and it becomes clear that she has become corrupt and may have killed everyone else in the center. Chell makes her way through the maintenance areas and empty office spaces behind the chambers, sometimes following graffiti messages which point in the right direction. These backstage areas, which are in an extremely dilapidated state, stand in stark contrast to the pristine test chambers. The graffiti includes statements such as “the cake is a lie” and pastiches of Emily Dickinson’s poem “The Chariot,” Henry Wadsworth Longfellow’s “The Reaper and the Flowers,” and Emily Brontë’s “No Coward Soul Is Mine,” mourning the death of the Companion Cube.
GLaDOS attempts to dissuade Chell with threats of physical harm and misleading statements claiming that she is going the wrong way as Chell makes her way deeper into the maintenance areas. Eventually, Chell reaches a large chamber where GLaDOS’s hardware hangs overhead. GLaDOS continues to plead with Chell, but during the exchange one of GLaDOS’ personality core spheres falls off; Chell drops it in an incinerator. GLaDOS reveals that Chell has just destroyed the morality core, which the Aperture Science employees allegedly installed after GLaDOS flooded the enrichment center with a deadly neurotoxin, and goes on to state that now there is nothing to prevent her from doing so once again. A six-minute countdown starts as Chell dislodges and incinerate more pieces of GLaDOS, while GLaDOS attempts to discourage her both verbally with a series of taunts and increasingly juvenile insults and physically by firing rockets at Chell. After she has destroyed the final piece, a portal malfunction tears the room apart and transports everything to the surface. Chell is then seen lying outside the facility’s gates amid the remains of GLaDOS. One of the final scenes is changed through a patch of the PC version that was made available a few days before Portal 2’s announcement; in this retroactive continuity, Chell is dragged away from the scene by an unseen entity speaking in a robotic voice, thanking her for assuming the party escort submission position.
The final scene, after a long and speedy zoom through the bowels of the facility, shows a mix of shelves surrounding a Black Forest cake and the Weighted Companion Cube. The shelves contain dozens of other personality cores, some of which begin to light up before a robotic arm descends and extinguishes the candle on the cake. As the credits roll, GLaDOS delivers a concluding report: the song “Still Alive”, considering the experiment to be a huge success.