Sam and Dean investigate a case in Chicago, where they discover that various mafia-esque monster families are, unknown to humans, running the underbelly of Chicago. One is a family of shapeshifters run by Margo Lassiter, whose leadership is thrown into question when her brother David returns home. The other reigning family in town is made up of werewolves, lead by Julian Duval. While in Chicago, Sam and Dean meet Ennis, a man with a personal vendetta against the monsters. The Winchesters warn Ennis about heading into the hunter lifestyle, but Ennis refuses to listen and starts down a dangerous path.