 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 . 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 destroys the robots . 
