Scott, Stiles, Lydia, Malia, and Liam are trying to figure out how to defeat the Beast of Gévaudan that the Maid of Gévaudan, Marie-Jeanne Valet, faced in 1765. As Gerard Argent, recounts Valet’s story to Scott’s pack in hopes they can deduce the creature’s weaknesses.
Stranded at a motel for the night because of the postponed cross country meet, Scott and the others suffer strange experiences that cause them to believe members of their own group may the mysterious killer’s newest targets.
With the threat growing far more personal and time running out on innocent lives, Scott is forced to make a difficult choice in order to save both friends and family while, at the same time, Derek and Peter try to figure out how to save Cora’s life.
On the night before Halloween, Mischief Night, a convicted murderer brought to Beacon Hills Hospital for surgery escapes, forcing Scott and his friends to step in to help when his unexpected target becomes clear.
Scott's friends pledge to protect him against an increasingly hostile enemy while Argent, Isaac and Allison search for answers through an old adversary from the Japanese Yakuza. At the same time, Stiles enlists Melissa's help in figuring out exactly what's going on with him.
Isaac's father is murdered; Scott, Stiles and Derek plan a break-out. At the same time, the Argent family remove the principal and Allison's grandfather replaces him. Scott senses that another werewolf is in the lacrosse locker room and plans to sniff them out, literally.
Even as his relationship with Allison becomes further strained, Scott and Stiles must attend Lydia's birthday party while Derek locks his new wolves up to watch after them on their first full moon together.
Scott races to uncover Gerard's master plan following a shocking death on the lacrosse field. Derek plots to take Gerard down using his own methods. Erica and Boyd are captured as Allison continues her merciless bloodthirsty revenge on the werewolves for her mother's suicide. Meanwhile, Scott, Stiles, Lydia, Melissa and Isaac are all under pressure amidst the chaotic events.