Spencer, Hanna, Aria and Emily discover the existence of Ali's institutionalized twin sister Courtney, who seeks to befriend them. She later reveals that she is actually Ali; she had gone up to her room after the fight with Spencer during their seventh grade sleepover, and Billy had killed Courtney instead. After the prom, Spencer, Hanna, Aria, and Emily go with Ali to her house in the Poconos, where it is revealed that Courtney had switched places with Ali when the girls had gone to steal Ali's time capsule flag, and had befriended them as Ali. The real Ali had also killed Courtney. Ali locks them in a room and sets the house on fire, trying to kill them in revenge for ruining her life. The girls escape and Ali is left inside. Spencer's mother and father consider getting back together. Hanna's mom moves back in and kicks Hanna's father, Kate, and Isabel out. She and Mike get back together. At the end it is hinted that Ali could still be alive.