|“|| "So Boyd and Cora might not have killed anyone?"|
"You're gonna wish they did."
"I'm not exactly sue yet. The other girl who was out in the woods, Emily? They're gonna find her. She's one of them. Emily, Heather, that guy Lydia found at the pool? They're all virgins, and they're all gonna have the same three injuries-- strangled, throat slashed, head bashed in. It's called the three-fold death."
"So if these aren't random killings, then what are they?"
"Sacrifices. Human sacrifices."
Scott finally confesses to Allison that her mother tried to kill him in Raving, while Derek discovers Erica's body in the supply closet of the bank. Derek, Isaac, and Scott seek help from Chris to capture Boyd and Cora before they kill someone. Lydia goes into a trance and inadvertently finds the body of a dead lifeguard at the swimming pool before calling Stiles for help. Scott, Derek, Isaac, and Chris (and with help from Allison, unbeknownst to all but Isaac) trap Boyd and Cora in the high school's boiler room before they discover that Jennifer Blake was inside as well.
After luring the rabid Boyd and Cora away from her until sunrise, when they promptly fell unconscious, Derek went to help Jennifer out of her hiding spot. After spending time helping Melissa McCall at the hospital, Stiles discovers Heather's body had been brought to the morgue with the same fatal injuries that the lifeguard Lydia found killed (head beaten in and strangled before their throat was slit, otherwise known as the "three-fold death" in ancient Celtic tradition). After realizing that Heather and the lifeguard were both virgins, Stiles concludes that someone is murdering them to offer as human sacrifices, just as Sheriff Stilinski discovers a third dead girl, Emily, killed in the same way.
- 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
- Ian Bohen as Peter Hale
- Sinqua Walls as Vernon Boyd
- Adelaide Kane as Cora Hale
- Haley Webb as Jennifer Blake
- Caitlin Custer as Heather
- Zelda Williams as Caitlin
- Lauren McKnight as Emily
- Mieko Hillman as Tara Graeme
- Anton Starkman as Billy
- Dani Fish as Breanne
- The title refers to the fireflies Argent and Scott saw out in the woods, which were a bioluminescent variety that is uncommon in California and were implied to be related to the recent supernatural phenomena in Beacon Hills. Interestingly enough, fireflies also play a role in the second half of Season 3 regarding the Oni.
- Lifeguard - three-fold death (strangled, throat cut, blunt force trauma to the head); killed by the Darach
- Heather - three-fold death; killed by the Darach
- Emily - three-fold death; killed by the Darach
- Beacon Hills Preserve
- Look-Out Point
- Martin House
- Lydia's Bedroom
- Beacon Hills Swimming Pool
- First National Bank
- Bank Vault
- Grocery Store
- Parking Lot
- Beacon Hills High School
- Ms. Blake's Classroom
- Boiler Room
- Bus Bay
- Parking Lot
- Beacon Hills Memorial Hospital
- Caitlin's Room
- "Ready to Go" by Guards
- Lydia rubs her temples in her bedroom and prepares to leave to go to the pharmacy for pain relievers for her headache
- "Do It" by Shana Halligan
- Emily and Caitlin make out and snuggle in an elaborately decorated tent in the preserve until Emily becomes freaked out by all of the bugs