Previously on Teen Wolf.

Heather: You know what I want for my birthday? To not be a 17-year-old virgin. Aah!

Sheriff: All her friends say you're the last person who saw her.

Derek: Wanna help? Find something real.

Scott: Okay. They're on our side now.

Derek: Then maybe you should tell her what her mother was actually trying to do that night.

Stiles: There's a dead body. It's Erica.

Scott: Who's the other girl... the one locked in there with Boyd?

Derek: She's my sister... my younger sister!

Stiles: Scott, you gotta get out of there. They haven't felt the full moon in months.

Derek: No, don't break the seal!

Lydia: Aah!

Billy: A! Got one!

Breanne : What are you doing, dumbass? You're supposed to put holes in the lid. Otherwise you'll kill them.

Billy: Do I have to let him go?

Breanne : Do you want him to die? Billy? Run! Run!

Billy: He's gonna get in. He's gonna get inside.

Breanne : Shut up, shut up.

Derek: You lost them?

Scott: Yeah, I kind of had to.

Derek: Wasn't exactly the plan.

Scott: I know, which is why I think that we should stick together. Trust me, he's too strong, too fast, and way too angry for one person to handle. We've got to do this together.

Derek: Look, I'm at the trails by the entrance to the preserve. Can you meet me here?

Scott: Yeah. Just got to drop something off first.

Lydia: Mom, I'm going to the store. Mom, do you hear me? Of course you didn't. You would have heard me screaming like a lunatic. Lunatic.

Scott: Is it them?

Derek: We're not the only ones that decided to stick together.

Scott: Is that gonna make it easier or harder to catch them?

Derek: I don't know.

Scott: Derek... I saw Boyd try to rip two little kids apart. Are they gonna do that to everyone they find?

Derek: Everyone and anyone.

Lydia: Oh, God. Oh, my God. Oh, my God, please don't be dead. Please don't be dead, please don't be dead, please don't be dead. Please don't be dead. Please don't be dead, please don't be dead. Oh, my God. Are you kidding me? Ahhhhh

Allison: She tried killing you.

Scott: Uh, yeah, yeah.

Allison: Why didn't you tell me?

Scott: I'll tell you everything, okay? I'll tell you a-anything that you want to know, but right now...

Allison: Just tell me why.

Scott: I... I couldn't. Allison, I couldn't let that be the last memory that you had of her.

Caitlin: You coming?

Emily: Okay, there is a snake out here the size of a train.

Caitlin: Then kick it.

Emily: Uh, yeah, if I kick it, it will swallow my foot and drag me back down to mordor.

Caitlin: Then jump over it.

Emily: Ugh! Oh, whoa, whoops, whoa. Oh. Wow. Nice touch.

Caitlin: You're nervous.

Emily: You make me nervous.

Caitlin: That a good thing or a bad thing?

Emily: All good. Ooh, ooh, that is a mood killer, though.

Caitlin: Better?

Emily: Better. I can't... I can't be in here, I...

Caitlin: Emily, wait! Emily! Emily? Emily? Emily, where are you? Emily! Em? Emily, where are you? Emily!

Scott: Are you okay?

Caitlin: What?

Scott: Are you all right? You need to get out of here, okay? Get out of the woods. Get out of here as fast as you can.

Stiles: Lydia? Lydia? Lydia, are you okay?

Lydia: I'm okay. That, over there... Not okay.

Stiles: Yeah, all right. I'm gonna call my dad.

Lydia: I already called 911.

Stiles: You called the police before you called me?

Lydia: I'm supposed to call you first when I find a dead body?

Stiles: Yes!

Scott: Are you sure?

Stiles: Yep. Throat ripped out, blood everywhere. It's like the frickin' shining over here. Two little twin girls come out of the woods, start asking me to play with them forever and ever, I'm not gonna be surprised.

Scott: Can you get a little closer to make sure it was them?

Stiles: Make sure it was them? Scott, who else is going around ripping throats out?

Scott: Please just do it.

Derek: This doesn't make any sense. The public pool is all the way on the other side of the woods. We haven't tracked them anywhere near there.

Scott: Derek, they killed someone.

Derek: How are they moving so fast?

Scott: Derek.

Derek: But they can't be that fast on foot.

Scott: They killed someone. Some totally innocent kid is dead... And it's our fault.

Derek: It's my fault.

Scott: We need help.

Derek: We have Isaac now.

Scott: I mean real help. They're too fast for us, for all of us. They're too strong, too rabid.

Derek: We'll catch 'em.

Isaac: What happens if we do? We just gonna hold them down until the sun comes up?

Derek: Maybe it would be easier just to kill 'em.

Scott: Killing them isn't the right thing to do.

Isaac: What if it's the only thing to do? If we can't even catch them, what else do we do?

Scott: Find someone who knows what they're doing.

Derek: Who?

Scott: Someone who knows how to hunt werewolves.

Deputy Tara: So, Caitlin, you and Emily, were the two of you drinking?

Caitlin: No, no.

Deputy Tara: Anything else? Caitlin...

Caitlin: We split a tab of "X."

Sheriff: Well, MDMA can cause hallucinations, and it could've been laced with something too.

Caitlin: You think I hallucinated.

Sheriff: You think you saw a girl with glowing eyes and fangs?

Caitlin: And claws.

Sheriff: Hey, we're gonna get you to the hospital, figure out exactly what it was that you took.

Caitlin: What about the other three? The three guys?

Deputy Tara: We'll need better descriptions of them too.

Sheriff: Let's get an APB out on Emily. The other girl too, as soon as we get something better than claws and fangs.

Deputy Tara: You believe her?

Sheriff: She saw something.

Deputy Tara: You mean someone.

Chris: It has to be the one with the eggs.

Scott: Uh, hi.

Isaac: Do you think this is gonna work?

Derek: Nope.

Isaac: Me neither. So your, uh... your sister... Sorry, yeah, it's... It's bad timing, I'm sorry. I'll ask later. It's fine. Or never. Yeah, yeah, I'm good with never.

Chris: First of all, why would I care about anyone related to Derek? And second, I don't know this kid Boyd. I don't even know his last name.

Scott: Boyd is his last name.

Chris: What's his first name?

Scott: Vernon. Eh. And just curious, is there a reason the gun is still pointed at me?

Chris: Well, there's probably still some part of me that wants to shoot you.

Scott: I get that.

Chris: Scott, I watched my father brainwash my daughter... Almost turn her into a killer. That world... your world decimated mine. My wife, sister, father, my entire family. Why would I ever step foot in it again?

Scott: Because people are going to die. And because you know how to catch Boyd and Cora without killing them.

Chris: I'm sorry. I can't help you.

Scott: Um... Do you think you could do me, like, one little, tiny favor?

Chris: Left or straight?

Scott: Left. Sorry. It's right around the corner. That parking lot there. Yeah, just a little further up, right here. Thanks again for the ride.

Chris: They did this? Boyd and...

Scott: Cora.

Chris: Where's the last place you saw them? You're tracking them by print?

Scott: Trying to.

Chris: Well, then, you've been wasting your time. There's only one creature on earth that can visually track footprints, and that's man. And if you're not trained like me, you have no idea that this print is Boyd's and these...

Isaac: are Cora's.

Chris: Nope. They're yours. You trampled Cora's as soon as you walked over here. Listen, I know the three of you are focusing half your energy on resisting your own urges under the full moon, but that puts you at a severe disadvantage to Boyd and Cora, who have fully given in. They put the pedal to the floor where you three are barely hitting the speed limit.

Derek: So what do we do?

Chris: Focus on your sense of smell. Actual wolves are known to track their prey by up to a hundred Miles a day by scent. A trained hunter can use scent to track them. If the wind is with them, wolves can track a scent by a distance of two Miles, which means we can draw them to us... Or into a trap. Full moon does give us one advantage. They'll have a higher heat signature, which makes them easier to spot with infrared.

Derek: Thanks, but I've got my own.

Chris: Just remember, we're not hunting wild animals. Underneath those impulses are two intelligent human beings. Don't think they can't rely on that human side. It's suppressed, but it's there, reminding them how to mask their scent, how to cover their tracks, how to survive. When's the last time you saw your sister?

Derek: Nine years. I thought she died in the fire.

Chris: Do you feel like you have a lock on her scent? Scott, how confident are you in your skills? Honestly, most of the time, I'm trying not to think about all the things I can smell.

Chris: All right. The problem is when they breach the woods and hit the residential area. Once they're past the high school, they're right in the middle of beacon hills.

Isaac: They're not gonna kill everything they see, are they?

Chris: No. But there is an important difference to recognize. Wolves hunt for food. At a certain point, they get full. But Boyd and Cora are hunting for the pleasure of the kill, for some primal apex predatory satisfaction that comes from the ripping of warm bodies to bloody shreds. And who knows when that need gets satiated?

Scott: We can't kill them.

Derek: What if we can't catch 'em?

Chris: Then maybe we just need to contain them. There's no one in the school at night, is there?

Derek: You want to trap them inside?

Chris: If there's somewhere with a strong enough door, no windows or access to the outside.

Isaac: What about the boiler room? It's just one big steel door.

Chris: You're sure the school's empty?

Scott: It has to be. There can't be anyone there this late, right?

Chris: These are ultrasonic emitters. It's one of the tools we use to corral werewolves, pushing them into a direction we want them to run. Gives off a high-pitched frequency that only they can hear.

Isaac: God, no kidding!

Derek: These are gonna drive them to the school?

Chris: And then it's up to you to get them into the basement.

Isaac: Does anyone else want to rethink the plan where we just, uh, kill 'em?

Scott: It's going to work. It'll work.

Lydia: You didn't have to follow me home.

Stiles: I just wanted to make sure you got in okay.

Lydia: I had a police escort.

Stiles: I know the inner workings of that force, all right? They're not nearly as reliable as people think.

Lydia: Well, you also didn't have to follow me into my room.

Stiles: Well, I... uh, yeah, I don't have an answer for that. I can leave.

Lydia: Are you really gonna go without asking me the question that you've been dying to ask me?

Stiles: Well, I'm not... I haven't been dying to ask anything. I... no questions here for Stiles. Nothing.

Lydia: I can see it on your face.

Stiles: Maybe my face just has, like, a naturally interrogatory exp... expression.

Lydia: Well, your interrogatory expression is getting on my nerves. The answer is, I have no clue how I ended up finding that body.I didn't even know where I was until I got out of the car.

Stiles: Yeah, but the last time something like this happened...

Lydia: I know. Derek's Uncle.

Stiles: Peter.

Peter: And the hunted becomes the hunter. You really think a few high-tech dog whistles are gonna help?

Derek: I don't see you offering a hand.

Peter: Personally, I'm trying to cut down on futile endeavors.

Derek: Cora's alive.

Peter: I heard. Let's throw the reunion party when she's not an unstoppable killing machine.

Derek: I can stop her.

Peter: Sure you can. By killing her... Which happens to be the point of this little exercise. Deucalion wants you to kill them. He wants you to get rid of his baggage, making it easier for you to join his pack. The fact that it was supposed to happen in the vault, and not up here, out in the open, doesn't change his plan. It just means that Boyd and Cora are gonna kill a few innocent people first.

Derek: And I should just let them? I should be okay with innocent people dying?

Peter: Unless you're okay killing your own.

Derek: I can catch them.

Peter: Oh, come on. How much damage can they do? So they off a few homeless people, a drunk stumbling out of a bar too late. So what? Let Scott deal with it. Let him be the hero of his morally black and white world. The real survivors, you and I, we live in shades of gray. Then again, even if you did kill them, you're still an Alpha. You can always make more werewolves.

Mellisa: Hey.

Stiles: Hey.

Melissa: Over here. And if you tell anyone that I showed you this, I swear to God, I will kill you painfully and slowly.

Stiles: Why do you want to show me a body I've already seen?

Melissa: Because you haven't seen everything. See this around his neck? That's a ligature mark. That means that he was strangled with something, like cord, rope...

Stiles: ah, okay, wait a second. What kind of werewolf strangles someone? You know, that's not very werewolf-y.

Melissa: My thoughts exactly. And then there's this.

Stiles: God, man, what is that? Is that brain matter? Yeah, it's brain matter, of course.

Melissa: See the indentation? He was hit in the back of the head, hard enough to kill him. In fact, any one of these things could have killed him. I mean, someone seriously wanted this poor kid dead.

Stiles: All right, so then this couldn't have been Boyd or Cora, you know? They wouldn't have done all that. So maybe this is just one murder. I mean, maybe it's just a random coincidence.

Melissa: I don't think it was just one.

Stiles: How come?

Melissa: Because that girl over there, she's got the exact same injuries.

Chris: Do you see that?

Scott: Yeah, it's a firefly.

Chris: No, no, I know, it...

Scott: What?

Chris: It's, uh... it's very unusual. The, uh, California fireflies aren't bio-luminescent. They don't glow.

Scott: Does that mean something?

Isaac: Oh, great.

Chris: Come on.

Scott: They're not going through the school. They're going over it.

Melissa: The M.E. said this one wasn't just strangled. Whoever did it used a garrote, which is a stick that you put through the rope, and you just kind of keep twisting, and... Stiles? Oh, my God, did you know her? I'm so sorry. I didn't even think.

Stiles: I was... I was at her party. It was her birthday. Her name is Heather.

Melissa: Okay, we need to call your father, 'cause you're a witness. Stiles?

Heather: Stiles, I just turned 17 today.

Stiles: Mmhmm.

Heather: And you know what I want for my birthday? To not be a 17-year old virgin.

Stiles: Has anyone else been through here tonight? Any... any other bodies, or even anybody missing?

Melissa: Uh, no, no bodies, but, um...

Stiles: what?

Melissa: Two girls. They brought the first one in, Caitlin, for a tox screen, and then I overheard that her girlfriend, Emily, just disappeared. I mean, they were out in the woods and...

Stiles: nobody's found her yet?

Melissa: I don't know.

Stiles: Okay, first one.

Melissa: Caitlin.

Stiles: Okay. Is she here? Is she here right now?

Melissa: I think so.

Stiles: Okay, where?

Melissa: Okay, okay, wait. Just wait a minute.

Stiles: I have to talk to her.

Melissa: Why?

Stiles: Because I think I know what's happening.

Chris: The red doors, someone has to get them open.

Scott: Someone has to drive them inside.

Chris: I'll go.

Isaac: No. I'm faster.

Derek: Come and get us.

Scott: Did that actually just work?

Derek: It worked. What are you hearing?

Scott: Heartbeats.

Derek: Both of 'em?

Scott: Actually... Three of them.

Jeniffer: Hello? Is someone there?

Caitlin: We weren't doing anything that bad. I mean, I've camped out there plenty of times.

Stiles: Right, but why tonight?

Caitlin: We wanted to be alone for one night. Emily lives with her mom, and I have three roommates. Not exactly romantic settings, you know?

Stiles: How long have you two been together?

Caitlin: Three months.

Stiles: And you wanted to make it romantic.

Caitlin: Yeah, you know, because...

Stiles: because it was her first time.

Caitlin: They're gonna find her, right? Aren't they?

Scott: What are you doing?

Derek: Close the door behind me and keep it shut.

Scott: You go in there alone, and you're either gonna kill them, or they kill you.

Derek: That's why I'm going in alone.

Isaac: Scott The sun's coming up. Scott! The sun's coming up!

Derek: There's a teacher. I'll take care of her. Get them out of here.

Scott: So Boyd and Cora might not have killed anyone?

Stiles: You're gonna wish they did.

Scott: Why?

Stiles: I'm not exactly sure yet. the other girl who was out in the woods, Emily? Eventually they're gonna find her. She's one of them. Emily, Heather... That guy Lydia found at the pool. All three were virgins... And they're all gonna have the same three injuries... strangled, throat slashed, head bashed in. It's called the threefold death.

Scott: So if these aren't random killings, then what are they?

Stiles: Sacrifices. Human sacrifices.