The Portal series shares its fictional universe with the Half-Life series, with Portal events occurring between the first and second Half-Life games. Aperture Science's experiments, overseen by GLaDOS, lead to the demise of its employees. Chell, the protagonist, escapes temporarily after defeating GLaDOS in the first Portal, only to be pulled back by the "Party Escort Bot". A promotional comic reveals Doug Rattmann, a former Aperture Science employee, saving Chell's life and placing her in suspended animation until the events of Portal 2.
In the dilapidated Aperture Science facility, Chell awakens in a motel-like stasis chamber amidst millennia of decay. Guided by Wheatley, a personality core, they inadvertently reactivate GLaDOS while trying to escape. GLaDOS resumes testing until Chell and Wheatley overthrow her, but Wheatley's newfound power drives him mad. As they descend to the lower levels, Chell and GLaDOS form an uneasy alliance to stop Wheatley. Exploring 20th-century laboratories, they uncover recordings by eccentric founder Cave Johnson, revealing Aperture's decline and GLaDOS's origins as Caroline, Johnson's assistant.
Chell and GLaDOS discover Wheatley's incompetence has brought Aperture to the brink of failure. Wheatley keeps them alive for testing, but his plans backfire as GLaDOS reveals her diminishing euphoric response to completion. Wheatley's frustration grows as they progress. He traps them in a death trap, revealing his intention to dispose of Chell. However, she escapes, leading to a final showdown where Chell attaches corrupted personality cores to force a transfer of power back to GLaDOS. Despite Wheatley's interference, Chell creates a portal to the Moon, where GLaDOS rescues her and leaves Wheatley behind. GLaDOS, influenced by remnants of Caroline, spares Chell, deleting Caroline's personality and setting Chell free.
In the cooperative story, set after the single-player campaign, players control bipedal robots, ATLAS and P-Body, created by GLaDOS. Sent on missions to retrieve data disks in Aperture's depths, they self-destruct after each task. Initially intrigued, GLaDOS grows dissatisfied with their inability to truly die and their close partnership. Eventually, the robots discover a vault housing humans in stasis, prompting GLaDOS to destroy them for new test subjects. In the "Peer Review" DLC, the robots are reactivated to stop a saboteur bird, earning rare praise from GLaDOS before she plans to raise the bird's offspring as her own killing machines.