Stiles' childhood friend, Heather, is attacked and goes missing. The guys help Isaac recover his memory, and it is revealed that he had found Boyd and another female werewolf, being kept in an abandoned bank vault. However, Erica may already have been killed. When Derek and Scott break in to save them, Derek recognizes the other female werewolf to be his little sister Cora, previously thought dead. Stiles and Peter discover that Boyd and Cora have been kept from transforming for 3 months. Wild and bloodthirsty, Boyd and Cora almost kill Derek and Scott until Allison saves them by setting the wolves free.