“Hello and, again, welcome to the Aperture Science computer-aided enrichment center.”

The first words heard upon starting the game, in a glass room. You are alone, with an AI named GLaDOS speaking, with overseeing your testing. Your only goal is to survive and escape the facility while avoiding obstacles.

Your only assistance is the Aperture Science Handheld Portal Device, with the ability to place two portals on special surfaces. 

The game ends with a battle against GLaDOS, in which you use portals and rocket turrets to detach her core systems, leading to the facility being heavily damaged, and your escape. This, however, is cut short, as you are dragged back into the facility.

Portal 2 begins with your character being awakened by a core named Wheatley, who sends you through the facility to help you escape.

Along your journey, GLaDOS is reactivated, taking back her job of testing. You assist Wheatley in replacing her, then she betrays you and sends you to the underground facility.

After you escape, you encounter Wheatley again, defeating him in a similar way to GLaDOS, however, being sent to space while GLaDOS retakes control, granting you freedom from the facility, ending the series.