Lydia goes missing from the hospital and both werewolves and hunters try to find her.
Isaac's father is cuddled the night before a full moon.
Scott tries to keep Derek from creating new werewolves; Argent officially begins Allison's training; the new creature in town takes another victim.
The shape-shifter continues his murders while Scott tries to get Allison to retrieve a rare book in Gerard's collection.
Derek is sure that Lydia is the shape-shifter, and asks Scott to help him find out.
Scott and Stiles uncover new evidence about the shape-shifter. In the mean time, Allison is given an assignment by her family.
Lydia begins to make connections about the bite from Peter, causing new problems for Scott and Stiles. Meanwhile, Derek has a crisis of faith in his leadership as a new alpha.
Scott and Stiles develop a plan to trap the new shapeshifter, while Allison's mom comes up with a plan of her own to take care of Scott once and for all.
The wolves that Derek has turned get prepared to go through their first transformation together on a full moon, and Derek locks them up so that he can observe the group and keep them safe. Meanwhile, at the same time that his relationship with Allison gets even rockier, Scott must go to Lydia's birthday party with Stiles.
While following Derek, Allison is lead back to the sheriff\'s station, where Stiles, Melissa, Scott, and the Sheriff are also being held hostage.
Scott has to pull double duty by playing in the championship lacrosse game and finding a way to stop Gerard.
The time is ticking as Scott tries to find out Gerard's master plan following a death on campus.