Portal 2

=== Setting ===
The Portal series shares a fictional universe with the Half-Life series. The events in Portal take place between the first and second Half-Life games, while most of Portal 2 is set "a long time after" the events in Portal.
Before Portal, Aperture Science conducted experiments to determine whether human subjects could safely navigate dangerous "test chambers", until the artificial intelligence GLaDOS, governing the laboratory, killed its employees. At the end of the first Portal, the protagonist Chell destroys GLaDOS and momentarily escapes the facility, but is dragged back inside by an unseen figure later identified by writer Erik Wolpaw as the "Party Escort Bot". A promotional comic shows estranged Aperture Science employee Doug Rattmann, who used graffiti to guide the player in Portal, placing Chell into suspended animation to save her life, until the beginning of Portal 2.

Plot

==== Single-player ====
In the Aperture Science Enrichment Center, player-character Chell wakes in a stasis chamber resembling a motel room. The complex has become dilapidated after what appears to be millennia of decay. Wheatley (Stephen Merchant), a personality core, guides her through old test chambers from the first game in an attempt to escape the facility after its reserve power is depleted. They accidentally reactivate the dormant GLaDOS (Ellen McLain) while attempting to restore power to the escape pods; she separates Chell from Wheatley and begins rebuilding the facility.
GLaDOS subjects Chell to new tests until Wheatley helps her escape again. They sabotage Aperture's production of turrets and neurotoxin to prevent GLaDOS from killing them, then confront GLaDOS and perform a core transfer; replacing her with Wheatley as the laboratory's controller. Wheatley, immediately driven mad with power, installs GLaDOS on a potato battery. GLaDOS tells Chell that Wheatley was designed as an "intelligence dampening sphere" to deliberately produce illogical thoughts and hamper her own intelligence. Infuriated, Wheatley inadvertently destroys the lift to the surface with Chell and GLaDOS inside, causing them to fall to the facility's abandoned lowest levels.
Chell retrieves GLaDOS in potato battery form and the two develop a reluctant partnership in order to stop Wheatley before his incompetence destroys the facility. Ascending through laboratories built in the 20th century, they discover audio recordings by eccentric Aperture Science founder Cave Johnson (J. K. Simmons). The recordings reveal how Aperture slowly lost money and prestige throughout the decades, as its pool of test subjects was altered from "astronauts, war heroes, and Olympians" in the 1950s, to homeless people in the 1970s, and Aperture's own employees in the 1980s. In 1981, Johnson became mortally ill after ingesting moon dust used to manufacture portal-conductive surfaces. His last request was for the mind of his assistant Caroline (McLain) to be transferred—by force, if necessary—to an advanced computer designed to store a human consciousness, which he had previously commissioned to save himself, creating GLaDOS. GLaDOS is troubled by the discovery that she is Caroline.
Chell and GLaDOS return to Aperture's higher levels, where they discover Wheatley's utter incompetence has brought the Aperture facility on the verge of catastrophic failure. Wheatley, after discovering Chell and GLaDOS had returned, decides to keep them alive as test subjects because the "franken-turret" cubes he created were ineffective at solving tests. GLaDOS later reveals that her body came built-in with a euphoric response to test completion that becomes less effective over time. Throughout the following chambers, Wheatley becomes increasingly frustrated with its dwindling effects.
Wheatley reveals that he has a "surprise" for Chell and GLaDOS, later tricking them into a death trap. He reveals that he had discovered the two robots from the Cooperative Testing Initiative, the game's cooperative campaign, making Chell no longer useful to him. She manages to escape from the trap right before its activation. The game's final levels are spent escaping from Wheatley's numerous attempts to kill them.
In the game's boss fight and ending, Chell confronts Wheatley and attaches corrupted personality cores (Nolan North) to force another core exchange and restore GLaDOS to authority. However, Wheatley demolishes the button necessary to initiate the transfer. As the facility crumbles, Chell places a portal on the Moon; she and Wheatley begin to be pulled into the vacuum of space. GLaDOS, having reasserted control over the facility, rescues Chell and abandons Wheatley to outer space. When Chell awakens, GLaDOS claims to have learned of humanity from the remnants of Caroline, but deletes Caroline's personality. Deciding that Chell is not worth the trouble of killing, GLaDOS frees her from the facility and gives Chell the burnt companion cube.

Co-op

The cooperative story is chronologically set after the events of the single-player campaign, but players are not required to play them in order. Player characters Atlas and P-Body are bipedal robots constructed by GLaDOS. In the first four 'sets' of levels, the robots are sent on ventures into the depths of the Aperture facilities to recover and upload data disks. After completion of each mission, the robots are disassembled and are reassembled to complete the next.
At first, GLaDOS is excited about her non-human test subjects, but later becomes dissatisfied because the two robots cannot truly die, and at one point also gets uncomfortable with their close partnership. At the end of the story, the robots gain entry to "the Vault", a storage facility of thousands of humans placed in stasis. GLaDOS thanks the robots for their acquisition of new test subjects, and promptly destroys the robots.

Peer Review

The robots are reactivated by GLaDOS one week after the original co-op campaign, during which GLaDOS has already wiped out all of the found test subjects after attempting to turn them into "killing machines". The robots are sent to find a saboteur that has taken control of a prototype central core and is causing problems in the facility. The saboteur is revealed to be a bird pecking at the console's keyboard, which sends GLaDOS into a panic when she recognizes it as the one who tried to eat her during her time as a potato battery. The robots manage to shoo away the bird, earning a rare compliment from GLaDOS before she notices eggs in its nest. Instead of having them smashed, GLaDOS has the eggs taken to her chamber so that she can raise the baby birds to be her killing machines.