Having discovers that the alpha pack got hold of his presumed-dead sister Cora, Derek searches her at all cost. There is a mysterious series of apparently random teenager killings, which fits Stiles's theory that virgins are targeted. Fearing the alpha pack, Scott and Derek plus Stiles and Isaac join forces, when Boyd and Cora become the prime suspects, with Peter and the Argents, Chris even teaches them to hunt werewolves. They track down the killer(s) to be lured to the school, which they presume abandoned, to be locked up in a basement vault section. Realizing someone was already there, Derek risks his life to save Jennifer Blake.