Two young kids are out catching fireflies, when one of them sees Boyd wolfed out. The boy grabs his sister and runs through the woods. They lock themselves in a cabin. The younger one talks about a male chasing them, saying that he’s going to get inside. Werewolf eyes are shown, suggesting that it’s Boyd. Boyd lifts the shed from over them and roars at the kids. The fireflies that they and been collecting swarm in his face, distracting him, and when he looks up the kids are gone. Derek and Scott talk on the phone, agreeing to look together after Scott reunites the kids with their family.
Lydia sits in bed, over tired and seeming stressed. She goes to take an ibruopofen before realizing that the bottle is empty and telling her mom she’s headed to the store.
Derek notices Boyd and Cora’s footprints, letting him and Scott know that they have stayed together.
Lydia gets out of her car to go into the store, but she’s texting and not paying attention, and when she looks up she’s at a pool deck. She gets a bad feeling and sees a body in the water, floating face down. She walks up to the pool, chanting “Please don’t be dead” under her breath as she flips the body over. It’s just a CPR mannequin. She backs away from the water, relieved, until she notices that her hands are covered in blood, and there’s a pool of blood next to her. There’s a man in the lifeguard chair who’s been murdered.
Allison sits in her car, remembering the conversation she had with Scott about what really happened the night Derek bit her mom. Her mom was trying to kill Scott and make it look like an accident, and the conversation is interrupted by Derek walking in with Erica’s body in his arms. Allison opens her glove compartment and pulls out an arrow head.
A brunette is setting up candles and lanterns in a tent while talking to her girlfriend. The two begin to kiss. One girl notices a huge bug, and her girlfriend slaps it away. Suddenly the entire tent is covered in bugs and younger seeming girl, Emily, runs out of the tent screaming. She’s followed by her girlfriend, but Emily has disappeared so the girl runs back. The bugs are shown attacking Emily until she disappears. Back at the tent, the remaining girl is approached by Cora, but then Isaac appears and attacks Cora. Scott and Derek appear as backup, and Cora runs, with Derek and Isaac right behind her. Scott stops to ask the girl if she’s okay and tells her to get out of the woods as quickly as possible.
Stiles drives up to the pool where Lydia found the body. Stiles is annoyed that Lydia’s already called the police. He gets in touch Scott and confirms that this boy was murdered the same way as the others. He’s wearing a purity ring. Derek doesn’t think that Cora and Boyd could have gotten all the way to the pool this quickly, but Scott isn’t buying it. Derek and Isaac say they might have to just kill them, but Scott suggests getting “someone who knows how to hunt werewolves” involved instead. The Sheriff and one of his deputies are discussing what happened to the remaining girl, Caitlin. She tells the police she saw a girl with glowing eyes and fangs, but she admits that she and her girlfriend had split a tab of MDMA, which is a known hallucinogen.
Chris Argent loads groceries into his car and pulls a gun on Scott, who is approaching him from behind. In the car, Isaac attempts to ask about Cora, but Derek shuts him downs with just a look. Although the conversation is tense at best, Chris eventually puts the gun away. He tells Scott how the supernatural has completely destroyed his family and his life. Scott tells him that they need him because it will save innocents. Chris refuses to help but does agree to give Scott a ride to the pool. Scott knows that Chris will see the victim first-hand, and as he predicted, Chris agrees to help and tells them that they’re footprint tracking is flawed.
Allison makes a small cut in her arm, knowing her blood will attract the Werewolf. Apparently she’s taking her own approach on this hunt without talking to the others. Chris Argent passes out infrared vision goggles, planning to find and trap the Werewolf somehow. Neither Scott nor Derek are confident that they can track either of them by scent, although that would probably be the most effective method. Chris suggests trapping them in the school if there’s a location secure enough, and Isaac suggests the boiler room. At the school, Ms. Blake sits at her desk grading papers.
Chris passes out sonic emitters, telling the Werewolf that they can use them to force Boyd and Cora to go in the direction they want, toward the school. Chris, Derek, Scott, and Isaac split up to plant the emitters leading toward the school.
Stiles follows Lydia home, and she tells him that she has no clue how she found the body. Both of them are thinking back to what happened to Peter.
Peter appears to harass Derek about his choices, and tells him that killing Boyd and Cora was Deucalion’s plan all along, and that he’s best off just letting Boyd and Cora do what they want.
Stiles walks into the hospital and meets up with Melissa McCall, who wants to show him the bodies that have been found so far. She points out the the bodies have all been strangled, had their throats slashed, and had fatal head injuries. Obviously Boyd and Cora didn’t kill him. There’s another body with the same injuries, a girl, making Melissa think it’s not an isolated incident.
At the school, Scott and Isaac put in place the final preparations to trap Boyd and Cora. Scott notices a firefly, and Chris notes that in California fireflies usually don’t glow. Boyd and Cora approach the school, and seeing Derek they decided to go over the school instead.
Melissa shows Stiles the other body, and it’s Stiles's friend Heather. He tells Melissa what happened. Apparently she was strangled with a garrote. At this point Stiles realizes that they were both virgins. Melissa tells Stiles about Caitlin and Emily.
Allison has brought arrows with flashes on them and uses them to get Boyd and Cora into the building. Isaac sees her, confused, and she bolts. Derek and Scott use themselves as bait to get Cora and Boyd into the boiler room. They spray fire extinguishers, hoping to confuse them before sprinting out of the boiler room and locking the door. Both Scott and Derek seem shocked that their plan actually worked, until Scott notices another heartbeat in the room. Ms. Blake is getting paper out of the supply closet that is attached to the boiler room.
Sitles and Melissa go talk to Caitlin. With Caitlin’s help, Stiles puts together that she wanted it to be a romantic evening, wanted things to be special for Emily’s first time.
Derek decides he’s going into the boiler room. Scott tells him that all three Werewolf won’t make it out, and Derek replies with “That’s why I’m going in alone.” Ms. Blake hears growling before she sees Cora and Boyd, but Derek attacks them before they can do anything to her. Derek stays in his human form and manages to hold them off, but they tear him apart, and he seems to pass out. The sun coms up and Isaac goes to tell Scott, who’s opened the door to go check on the Werewolf. Derek is worn out and torn up, but alive, and both Boyd and Cora are passed out at his feet. Derek tells them that there was a teacher, but he’ll take care of it. Derek goes to talk to Ms. Blake, approaching her carefully and offering her a hand.
Stiles takes Scott to the hospital and shows him what’s happened with the bodies before revealing his theory. The Sheriff finds Emily, killed the same way as the guy at the pool and Heather. All three were virgins. Apparently this method of killing is a sacrificial procedure called the threefold death.