The cooperative story takes place after the single-player campaign and has some ties into it, but players are not required to play them in order. Player characters Atlas and P-Body are bipedal robots who navigate five sets of test chambers together, each with a fully functioning portal gun. After completing a test chamber, the robots are disassembled and reassembled at the next chamber. After completing each set of chambers, they are returned to a central hub. The puzzles in each set of chambers focus on a particular testing element or puzzle-solving technique. In the first four sets, GLaDOS prepares the robots to venture outside of the test systems of Aperture Laboratories to recover data disks. She destroys them and restores their memories to new bodies—which also happens when they die in a test chamber hazard. 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", where humans are stored in stasis. GLaDOS gives thanks to the robots on locating the humans, whom she sees as new test subjects, and the game ends.