Portal 2 comprises a series of puzzles that must be solved by teleporting the player's character and simple objects using the "portal gun", a device that can create inter-spatial portals between flat planes. The game's unique physics allow momentum to be retained through portals, requiring creative use of portals to maneuver through the test chambers. Other gameplay elements were added to Portal 2, including tractor beams, laser redirection, and special paint-like gels that impart special properties to objects they touch.
Within the single player campaign, the player returns as the human Chell, having awakened from stasis after several hundred years. Chell must navigate the dilapidated Aperture Science Enrichment Center and its test chambers with the portal gun while the facility is rebuilt by the reactivated GLaDOS, a rampant artificially intelligent computer. With a larger story, Valve introduced additional characters, including Wheatley, voiced by Stephen Merchant, and recordings of Aperture Science CEO Cave Johnson, voiced by J.K. Simmons. Ellen McLain returns to voice GLaDOS. Additional music from Jonathan Coulton and The National appear in the game. Portal 2 also includes a two-player co-operative mode in which the player-characters, the robots Atlas and P-body, must work together to complete each test chamber using their own individual portal guns.
Though many reviewers were concerned about the difficulty of expanding Portal into a full sequel, critics universally praised Portal 2. The game's writing, pacing, and comedy were highlighted as stand-out elements, with critics applauding the voice work of McLain, Merchant, and Simmons. Reviews also highlighted the new gameplay elements, the game's challenging but surmountable learning curve, and the addition of the co-operative mode.
PlotEditPortal 2 takes place after the first game as part of the Half-Life series, some time after Half-Life 2. Despite her apparent destruction at the end of Portal, GLaDOS, an artificial intelligence computer system, remains functional but dormant. The player controls Chell, the previous protagonist of Portal. Retroactively patched just prior to the sequel's official announcement, the ending of the first game shows Chell being dragged away by an unseen figure with a robotic voice, later identified by writer Erik Wolpaw as the "Party Escort Bot", after which she is placed in stasis for a period of many years. The game again takes place in the Aperture Science Labs, untouched by human hands but overrun by decay and nature. One hidden area of the game shows the former location of Aperture Science's cargo ship, the Borealis, in the bowels of the facility; this ship is mentioned as part the Half-Life universe at the end of Episode 2, having been discovered in the Arctic, situated in a seemingly-impossible position with all hands lost. Portal 2 suggests the ship was related as a result of an experimental teleportation device being developed by Aperture to compete with Black Mesa Research Facility, a competitor featured in Half-Life.
Parts of the story are bookended by the "Rat Man", an unseen character in the first game whose background is fleshed out in an online comic "Lab Rat" produced by Valve to tie the history of the games together. Rat Man, a scientist at Aperture, escaped from the neurotoxin released by GLaDOS that wiped out the rest of the Aperture staff, and slowly became insane. However, he sees that Chell is unique for her tenacity among the thousands of test subjects stored by Aperture, and seeks to protect her, moving her to the top of the list subject queue to allow her to escape. He wrote messages on walls to guide and warn Chell during the events of the first game; having witnessed GLaDOS's defeat at the end of the first game, the Rat Man's drawings created in the months after the events summarize the first game during Portal 2's introduction. Rat Man, though able to escape after GLaDOS's destruction, returns to the facility following the robot that drags the unconscious Chell back inside. As a final act, he ensures that Chell is kept in indefinite cryostorage, before he himself enters a cryogenic unit after being severely wounded by turrets.
Chell wakes to find herself in what appears to be a motel room. A robotic voice guides her through a cognitive test before she is put back to sleep. When she next awakes, many years have passed. Wheatley (Stephen Merchant), a personality core, moves the room—located in one of hundreds of shipping containers among a giant warehouse—helping her to escape through the test chambers of the Aperture Science facility. Wheatley has become concerned about the state of decay and seeks to correct it, guiding Chell through the test chambers to the dormant GLaDOS (Ellen McLain). While attempting to activate an escape pod, they accidentally wake her. GLaDOS has not forgiven Chell for "murdering" her years ago, and begins rebuilding the ruined facility to put Chell through more tests, stating, "I think we can put our differences behind us... for science... you monster."
With Wheatley's help, Chell is able to escape the test chambers and disable the neurotoxin and turret manufacturing plants. When they confront GLaDOS a second time, Chell is able to exchange Wheatley's personality for GLaDOS's. Wheatley becomes intoxicated with power and places GLaDOS's personality into a module powered by a potato battery; this form of the AI is nicknamed "POTaTOS" within the developer commentary. He betrays Chell and sends both Chell and GLaDOS miles deep into the bowels of the facility. As they fall, GLaDOS chastises Chell for putting Wheatley in the position of power, claiming that he was "the product of the greatest minds of a generation working together with the express purpose of building the dumbest moron who ever lived", designed to hamper GLaDOS's decision-making processes.
After landing, GLaDOS is abducted by a bird, while Chell works to climb back to the higher levels through a series of chambers automated by recordings of Aperture Science's CEO, Cave Johnson (J. K. Simmons). During her climb through the various strata of old Aperture test chambers, Chell learns from the recordings that Johnson became increasingly deranged over time. His assistant, Caroline (McLain), would eventually be a test subject for a mind-to-computer transfer system and be placed in charge of the facility, ultimately becoming part of GLaDOS' personality makeup. Chell and GLaDOS are reunited, and partner to stop Wheatley before his incompetence destroys the complex.
Chell and GLaDOS eventually reach the main facility again and face Wheatley who puts them through further test chambers, being driven to do so as a result of hardwired programming. In a final "surprise", Wheatley tricks Chell into a series of death traps, GLaDOS denoting that "this is the part where he kills you", but Chell deftly escapes from Wheatley's clumsiness, and makes her way to Wheatley's chamber. In their final confrontation, Chell distracts Wheatley by adding defective core modules in order to allow GLaDOS to initiate another core transfer and put herself back in control of the facility. As the facility's roof collapses, Chell shoots a portal at the moon, sucking her and Wheatley into space. GLaDOS manages to pull Chell back inside while leaving Wheatley to be stranded. Some time later, Chell wakes up, and GLaDOS explains that she learned valuable lessons about humanity from her Caroline persona. She then deletes this aspect of her personality, reverting to her standard antagonistic attitude, but finally allows Chell to leave the facility, explaining that trying to kill her has proven so difficult that it's easier to just let her go. The game ends as Chell is taken to the surface, and after a brief interlude of being serenaded by a choir of turret guns, exits from an unadorned shed into a wheat field; the charred and battered Weighted Companion Cube, supposedly incinerated from the events in Portal, is spit out from the door before it slams shut. In the epilogue, Wheatley floats aimlessly through space and expresses regrets about betraying Chell.
Two new characters are introduced in the two-player cooperative mode, which has its own unique plot and setting. These two characters are Atlas and P-body, a modified personality core and turret gun, respectively both units are bipedal and equipped with their own portal guns. Though once part of the networked facility, they have become separate entities and are treated to similar abuse by GLaDOS through a series of complicated test chambers through her "Cooperative Testing Initiative". The robots make "expressive noises" in place of distinguishable dialogue. The robots' mannerisms were designed to be a double act similar to Laurel and Hardy. GLaDOS appears to be troubled by the robots working together, and will attempt to aggravate their relationship through verbal trickery such as praising one robot over the other. Though the co-op story takes place chronologically after the single player campaign and has some ties into it, Wolpaw claimed that players do not "necessarily need to play them in that order".
In the co-op campaign, Atlas and P-body are shown to five sets of test chambers by GLaDOS. In the first four of these, GLaDOS prepares the robots to "venture outside" of the test systems of Aperture Laboratories to recover a data disk, after which she destroys them, restoring their memories to new bodies, as it is the only way she can recover them from "outside". Collectively, the data points to the location of "the Vault", where GLaDOS claims humans can be found. At the conclusion of the fifth test chamber, the robots discover and gain entry into the Vault, a giant structure where thousands of human bodies in cryogenic suspension are found. GLaDOS gleefully congratulates the robots, seeing the humans as new test subjects for her to process.