Tier 3 is an upcoming mod for Portal and Portal 2, it will take place in the year or so before Portal. Kat, the main character, is an elite test subject raised for the sole purpose of being the perfect test subject. Over her many years of testing, she has exhibited the kind of behavior GLaDOS hoped for. However, the Aperture Science Handheld Portal Device is nearly finished and Kat yearns to see the outside. After all, GLaDOS won't need her after she can finish the ASHPD, right?