|“|| "What are you doing?"|
"What's necessary. I'm still surprised none of you seem to get that. You call them sacrifices, but you're not understanding the word. It's derived from the Latin sacrificium. An offering to a deity. A sacred rite. A necessary evil."
"Oh, I wish I could. But you don't know the Alphas like I do."
"But you, Lydia? You're not a sacrifice. You're just a girl who knows too much. Actually, a girl who knew too much."
Allison and Isaac learn that the Darach's trio sacrifices include "philosophers" and "guardians" in addition to "virgins," "warriors," and "healers" after finding a map in Allison's father's office. This leads the two to believe that Chris Argent is involved with the murders, but Allison and Isaac end up discovering that he has actually been tracking the Darach since the murders began. Meanwhile, Scott and Stiles want Lydia's help to solve the murders. Lydia discovers that Jennifer Blake is the Darach. Jennifer is about to kill her for knowing too much, but when Lydia instinctively screams in fear, Jennifer realizes that Lydia is actually a Banshee, aka "the Wailing Woman," explaining the fact that she is unconsciously drawn to death and the supernatural and has premonitions of future deaths to occur. Stiles' dad tries to save Lydia, but is kidnapped by Jennifer, as his role as a parent and the Sheriff of Beacon County made him a fitting choice for the guardian sacrifice trio. Scott goes to Ms. Morrell for some guidance, where she reveals that Deucalion's plan for him is to either manipulate Scott into willingly joining the Alpha Pack after rising to True Alpha status, or Deucalion will force Scott to kill, destroying his potential to become a True Alpha.
- Tyler Posey as Scott McCall
- Crystal Reed as Allison Argent
- Dylan O'Brien as Stiles Stilinski
- Tyler Hoechlin as Derek Hale
- Holland Roden as Lydia Martin
- JR Bourne as Chris Argent
- Melissa Ponzio as Melissa McCall
- Linden Ashby as Sheriff Stilinski
- Daniel Sharman as Isaac Lahey
- Bianca Lawson as Marin Morrell
- Charlie Carver as Ethan
- Max Carver as Aiden
- Adelaide Kane as Cora Hale
- Haley Webb as Jennifer Blake
- Keahu Kahuanui as Danny Mahealani
- Ariel Flores as Student
- Mieko Hillman as Deputy Tara Graeme
- Meli Alexander as Teacher (uncredited)
- Laura Bedwell-Young as Beacon Hills Student (uncredited)
- Alex Borquez as Orchestra Member (uncredited)
- Roni Modlin as Choir Member (uncredited)
- Tina Q. Nguyen as Beacon Hills Student (uncredited)
- The title refers to Lydia Martin, who has unknowingly been drawing the Nemeton in a brief Banshee fugue states all season, which Jennifer sees. Jennifer then decided to kill Lydia, stating, "You're just the girl who knew too much."
- The conductor at the school's recital is played by Dino Meneghin, who composes the scores for Teen Wolf.
- Mr. Westover - three-fold death; killed by the Darach
- Recital pianist - slit throat; killed by the Darach
- Beacon Hills High School
- Locker Room
- Parking Lot
- Ms. Blake's Classroom
- Coach's Office
- Guidance Office
- Mr. Westover's Classroom
- Argent House
- Allison's Bedroom
- Chris' Office
- Stilinski House
- Stiles' Bedroom
- Electric Substation
- Beacon Hills Memorial Hospital
- Emergency Room
- Reception Area
- Cora's Room
- "Gentle Folks" by Hunter Hunted
- Jennifer is teaching her English class at the high school, where she lectures about idioms and cliches.
- "Better With You" by of Verona
- Lydia brings Aiden into Coach's office to make out and get answers about the Alpha Pack.
- "Let's Go" by Korn ft. Noisia
- Aiden, thinking Derek is luring him into the locker room, goes to find him and finds that it's actually Cora who wants to attack him.
- "I Can't Find You" by SolarSolar
- Jennifer rushes to the tunnel leading to the lacrosse field to meet with Derek, who she hasn't seen since Boyd's death.