In a flashback, it's a full moon and the Beacon Hills basketball team is heading out to play. Derek is lagging behind, still in the locker room. Peter goes in and finds him partially shifted and running cold water over himself to calm down. Peter shuts off the water and yanks Derek up from the floor. Derek tells him that he thought he could control the shift. Peter scoffs at the risk he's taking just for a basketball game. "It's the finals. They need me." "To do what? Rip someone's throat out on the court?" Derek stumbles around in pain. Peter says that even born wolves have to learn to control themselves on a full moon. Derek replies that he thought it was supposed to be easier for them and asks why it hurts so much. Peter demands to know if Derek brought "it." Derek tells him it isn't working, but Peter insists, and Derek pulls out a triskelion medallion. He starts to chant "alpha, beta, omega" trying to focus. He throws his arms wide and yells toward the sky.
In present time, a gas station attendant, Briggs, sees a car seemingly abandoned by the pumps. He hears some screaming and growling from the bathroom and goes to investigate. Kate is inside trying to control her shifting. The attendant keeps banging on the door and ordering her to open it. He eventually opens the door himself, and Kate turns, shifted. She attacks the guy and eviscerates him. Scott, Stiles, Malia, Lydia, and Kira drove all night from Mexico to bring Young Derek directly to Deaton at the Animal Clinic. Scott carries him inside, and they lay him out on an exam table. "Wow," Deaton says. "Wow. Wow as in 'I've seen this before and I know exactly what to do,' kind of wow? Cause that's the kind of wow we were hoping for," Stiles says. Deaton has no idea what to do. Lydia takes Young Derek's hand and notes that he's really cold. Deaton starts trying to check his vitals. Scott asks if this is permanent, and Deaton replies that he's not sure a medical diagnosis is even adequate.
Stiles asks what they should do, and Deaton suggests they leave Derek with him, since there won't be much to do until he wakes up. The mountain ash of the clinic can keep Kate out. Lydia asks why Kate would want to make Derek young again. Deaton can only say that whatever the reason, it must only benefit Kate and be bad for the rest of them. He suggests that they all go home and get some sleep, because it's a school night. "You all need to start taking care of your own lives again." Scott thinks someone should stay. Young Derek squeezes Lydia's hand, and she volunteers. Her grades are good, so she can miss a class or two. Stiles exclaims that he is so not okay with this plan. Lydia insists they leave. Scott tells her to text them if anything happens, and Stiles says, "No, still not okay with it. Not going anywhere." Scott drags Stiles out of the office.
Scott returns home to find the roof leaking. There's a pot on the floor catching the drops. He tries to sneak around but sees his dad asleep on the couch and gets distracted. He kicks a toolbox, making a racket, and wakes his father up. He asks what's up with the tools, and Rafael tells him that he's been fixing things up around the house. Rafael asks him what time it is, and Scott lies and says midnight. It is 4am. Rafael and Scott have established a new routine. When Melissa has a night shift, they are supposed to have dinner together. Scott missed a dinner by going to rescue Derek. Scott explains that when they got back from their camping trip he went straight to the animal clinic to finish up work.
The next morning, Lydia wakes up in the Animal Clinic to find Deaton taking Derek's pulse. He reports that it's alarmingly high. Derek isn't as cold as he was. Deaton asks Lydia to keep holding Derek's hand while he cuts into his skin. The wound healed as quickly as he made the cut, which is much faster than usual. He wants to try something else, but while they're getting the syringe, Derek wakes up. His vision is confused and senses muffled. All he can see are their vague outlines and their eyes. Lydia and Deaton keep saying his name, but Derek doesn't recognize them. Frightened, Derek claws Deaton and runs away.
At school, Stiles explains to Scott that at 2am he'll wake up and Malia will be in his bed next to him. She sneaks in. "This was like five times a week." Scott asks what happens then, and Stiles pulls up his shirt to show Scott scratches on his back. Stiles doesn't sound thrilled. They then spend the rest of the night spooning, but Stiles is always the little spoon, which he also doesn't sound happy about. Scott asks if that just means they're together, that they're dating. Stiles skirts the question and replies that the way she looks at him sometimes it's like she knows he's not telling her something. Stiles thinks they need to tell her about Peter, but Scott doesn't know how.
In history class, Mr. Ken Yukimura tells them that some of history's greatest leaders have had to endure some pretty great failures. He describes how Abraham Lincoln failed in business, had a nervous breakdown, was defeated for both houses of Congress, and lost as vice president. Mr. Ken Yukimura asks who this president was, and Stiles raises his hand. He winks at Scott, but Mr. Ken Yukimura calls on Malia instead. She has a highlighter in her mouth and another pen in her hand, highlighting something, or perhaps coloring. She stares at the teacher when he calls on her, not knowing the answer. Just then Scott's phone rings. Mr. Ken Yukimura orders him to turn it off and keeps prompting Malia. Kira waves her hand anxiously, trying to be called on. Stiles's phone chimes next, and Mr. Ken Yukimura again orders everyone to turn their phones off. When Malia can't answer, he asks if anyone else knows, and everyone raises their hands. Another phone chimes, prompting an angry reply from Mr. Ken Yukimura, but Kira points out to him that it's his phone. He checks the message. "Scott, call Lydia." Scott and Stiles head over to the Animal Clinic, where Lydia is patching up Deaton's arm. "I don't think he's just younger in body. I think he's younger in mind too," Deaton tells them. Stiles asks where a scared teenage werewolf would go. Scott says that a wolf goes back to its den, but Derek lives in a loft. "Not when he was a teenager," Stiles points out. They realize that he'll go to the Hale House. He wouldn't remember the fire because it wouldn't have happened yet.
Young Derek breaks into the construction site where his house once stood. The burned out husk has been knocked down. Derek picks up one of the boards.
Lydia stops Scott and Stiles before they leave and asks what they intend to say to Derek when they find him. Scott thinks he'll have to tell him the truth: that his whole family is dead. "Oh, good luck with that," Lydia replies, nodding. Stiles agrees with Lydia and suggests not telling Derek about his family until they know how to get him back to normal. "I can't lie to him." "Okay, I'll do it," Stiles says. But what Scott means is that Derek is a werewolf and can tell a lie. Deaton points out that if Derek gets to the house first they won't have to tell him the truth.
Back at the ruins of the house, Derek is visibly upset. Deputies Parrish and Haigh pull up because somehow they knew he broke into the site. Parrish tells Derek that he can't be there. "Hey dumbass, you ever hear of no tresspassing?" Haigh asks, putting himself forever on fandom's shitlist. Derek replies that this is his house, but Haigh tells him that no one's been there for years. Parrish plays nice cop and asks Derek if they can help him. "What happened to my house? Where's my family? Where's my mother?" Haigh grabs Derek and pulls him up, and Derek responds by grabbing Haigh's arm hard. Parrish tries to stop the confrontation before it gets too bad, but Haigh takes the distraction opportunity to taser Derek. They take him back to the station. Parrish comes over and tells Derek that he thinks if they take his cuffs off he'll be okay and help them figure out what happened to his family. Derek calls Deputy Haigh an idiot. "Can't argue with that," Parrish says, smiling brightly. Haigh calls Parrish over because he's run Derek's prints 8 times and all that comes up is the record for Derek looking a whole lot more adult. The Sheriff walks by then and looks at the screen. He looks at Young Derek, then the screen. He goes over and crouches in front of Young Derek, getting a closer look, then checks the computer screen again. Just as he's considering the options Stiles and Scott come running into the station. "I'll handle this," the Sheriff tells his deputies.
In his office, the Sheriff tells Scott and Stiles that he wants them to be honest. "Absolutely and completely honest. Have you been time traveling?" "Hang on, what?" "Because if time traveling is real, you know what? I'm done. I'm out. You're going to be driving me to Eichen House." Scott claims that they found Derek de-aged. "Where? Swimming in the Fountain of Youth?" Stiles replies, "No. We found him buried in a tomb of wolfsbane in an Aztec temple in Mexico underneath a church in the middle of a town that was destroyed by an earthquake." Scott looks unimpressed that Stiles chose now to tell the truth. "You told me you were camping!" the Sheriff says, dismayed. Stiles replies that they were . . . in Mexico.
Scott just wants to talk to him. The Sheriff says he isn't talking to anyone, but Scott thinks he can get him to open up. He explains that there was an accident and Derek lost his memory but they can help him get it back. While Scott is talking to Derek, Stiles sees an overdue bill from Eichen House on his father's desk. Scott assures Derek that he can trust him and then flashes his red eyes. Derek demands to know who Scott and Stiles are, and Stiles responds that they're the guys keeping him out of jail. Stiles points out that he almost tore apart two cops, so he has to start listening to them. No fangs, no claws, no wolf man. Derek tells them that he's fine so long as it's not on a full moon. Scott is surprised that Derek can't control himself on a full moon yet. Stiles asks if he's going to come with them, and Derek asks them where his family is. Scott decides to lie. "There was a fire and... They're no here anymore. They're fine." The sound of his heartbeat is played loudly to show how steady it is while he lies. "Just had to move out of Beacon Hills. And we're going to take you to them as soon as we figure out how to get your memories back." Afterward, Scott feels bad about it, but Stiles thinks he saved Derek a ton of unnecessary pain. When they figure it out in a day or two, everyone will go back to being happy, except for Derek who's never happy. Scott says that he thinks it's better when they tell the truth. Scott has Stiles take Derek to his house to watch him. Scott is going to talk to Peter, which Stiles does not approve of.
Lydia and Kira pull into the gas station to fill up. Kira reports from the text she just got that they're supposed to go meet Stiles at Scott's house. Lydia thinks going to Peter is a horrible plan but at least they've had worse. Kira asks Lydia about yesterday, when she and Scott were tied up to electrical cables. She wants to know if Lydia would blame her if she'd had to shock her at the Calaveras' command. Lydia assures her that Scott knows it wasn't her fault and that he's totally into her. Lydia gets out and tells Kira to grab a credit card from the visor so they can fill up. While Kira is looking at the cards, she realizes that the tank is full. She gets out and finds Lydia standing next to the bathroom, staring. Lydia has found another body. She warns Kira not to look, but she looks anyway. There's blood and entrails everywhere.
It takes Scott a long time to get to the loft, because it's night when he arrives. He finds Malia waiting for him. He asks what she's doing there. "I heard you were coming to talk to Peter. And since Lydia tells me he's basically Satan in a V-neck, I figured you shouldn't be alone." Scott claims he can handle Peter, but Malia isn't taking no for an answer.
At Scott's house, Stiles announces that they're going to wait there and sit quietly, and they're not going to call or talk to anyone. "Do I talk to you?" Derek asks. "No." "Good." "Fine." "Who's going to talk to him?" Derek asks, pointing at Rafael McCall. Stiles flails to a halt. "Are you getting taller?" McCall asks them what they're doing there. Seems that they're all waiting for Scott, because they're supposed to have dinner. He offers them food, which Stiles tries to decline and Derek is all about. "No, I'm starving," Derek says. "Neither of us are hungry," Stiles insists. Agent McCall offers Derek food since he's hungry and asks his name. Derek starts to answer, but Stiles says, "Miguel. My cousin Miguel. From Mexico." He pats Derek on the shoulder, and Derek plays along. Agent McCall then asks Derek if he's originally from Mexico in Spanish "Es usted un natural de México, Miguel?" "Oh my God," Stiles mutters, sure he's been caught in his lie. Derek answers in Spanish, "No soy nativo, si no que pasé montón de tiempo allí." He's not a native but he spends a large amount of time there. Agent McCall seems happy with this response and asks if Derek wants an egg roll. "Hell yeah," Derek replies with a smile. Stiles is thrown but thrilled.
Scott and Malia head into the loft. She asks him what's wrong with him and if he's nervous because his heart is pounding. "He's just bad at introductions," Peter says from the couch. Scott introduces them, and Peter sets aside the book he was reading with new interest. He asks Malia if she got her eyes from her father. She says her mother. He takes a few steps closer, looking at her, and then changes the subject. Malia admits that the killing spree came up in conversation. Peter repeats a line he said to Derek once, "We're all works in progress." "Well when you progress to your next killing spree, why don't you try to make sure they all stay dead?" Peter isn't sure what she means by that. Scott asks him about someone being turned by a scratch, prompting Peter to ask if he scratched someone, but Scott gives him a look that says the question isn't about him. "But it's possible. Like if you clawed out someone's throat?" Scott asks. Peter puts the pieces together and does not look amused.
Agent McCall asks Miguel what his last name is. Stiles supplies Juarez Cinqua Tiago. Derek looks at him like he's an idiot. McCall comments that that's a mouthful and asks how you spell it. Derek then gives Stiles an amused and expectant look. "Phonetically," Stiles replies. While they're eating, Derek sees McCall's FBI badge and asks him whether he investigates fires. Stiles gets very anxious and proclaims to wonder where Scott is. He starts calling Scott while Derek asks McCall if he knows anything about the Hale family.
Peter growls, "Can't someone in this town stay dead?" Malia answers with, "I think they were hoping you would." Scott asks if he can come up with a reason why Kate would turn Derek into a teenager. Peter asks him what color Derek's eyes were. The fact that they were blue means he's aged to after he killed Paige but before Kate betrayed him and killed his family. Scott learns that Derek and Kate knew each other before the fire, which means that Stiles kept this information from Scott. Peter tells him that they knew each other "Biblically. That's right, Scott. You weren't the first wolf to climb into a hunter's bed."
Young Derek slams Stiles against the door of Scott's room. Stiles protests that he didn't lie so much as omit certain truths. We actually haven't seen him lie at all. Derek lets him go and says he wants to talk to Scott. Stiles agrees to get him. He's going to go downstairs and get his phone. He tells Derek to stay and does a quick double-take to make sure he doesn't leave the room before heading down.
Scott tells Peter that Derek thought the house was still there and doesn't remember the fire. This means that Derek is at a point in his life when he still trusted Kate. The door to Scott's room pushes open, and Kate walks in. "Hey, handsome," she says to Derek. Stiles comes back, on the phone with Scott. "To be honest, I'm starting to miss the old Derek." As he gets back to Scott's room, he sees Kate outside Scott's now open window and no Derek. Peter and Malia start running for the door, but Scott calls for them to wait. Peter asks, "For what? Kate's out there twisting her way into Derek's head yet again. We need to find her." Malia says all they need is a scent. Scott thinks that could take hours. Instead they need to figure out where she's going. Peter says they're not exactly a brain trust of genuises, so Scott calls Lydia.
Lydia and Kira have apparently been standing at the gas station for hours. Lydia reports that they called the Sheriff and he's on his way. Scott asks her to describe the scene and send pictures. She and Kira are understandably squeamish. Peter declares that from the level of violence, it was a frenzy, not a murder. Kate can't control the shift. Malia asks if that means Kate wants Derek to teach her, but Peter realizes that Kate wants the Triskelion.
Derek takes Kate to the high school. He hesitates, though, because he's not supposed to show the vault to anyone outside his family. She wheedles him and kisses him to convince him to do it. She says all they have to do is get into the vault, get the Triskelion, and everything goes back to normal. He takes her to the school sign. Kate asks him why they would build a vault under a high school, but Derek says the vault was there first. He inserts his claws into a dial on the side of the school sign and turns it. The sign swings around sideways revealing a staircase, and Kate smiles. Once they're down in the vault, Kate focuses on a safe in the middle.
Scott, Peter, and Malia are close behind. Malia stops and claims that she caught a scent, the same one that attacked them in Mexico. Scott thinks they could have been brought by Kate. One of them growls, and Peter looks unamused. "Oh, I've heard that sound before." Peter tells them that the creatures are Berserkers. One of the Berserkers steps toward them, and Malia gets ready to fight. Peter stops her. "Are you crazy?" She says there's just one. "That means we have a chance." "To beat him?" "To survive." Then Peter takes off running. Scott and Malia wait a few seconds before they start running too.
Inside the vault, Derek finds the Triskelion. When he turns around, he sees Kate going for the safe. "Not that." He brings her a case and opens it to reveal the Triskelion. Scott and Malia run from one Berserker only to find a second blocking their path. They chase them up the stairs and have them surrounded.
Kate takes the Triskelion out of the case, claiming that it doesn't look like much. "That's cause it isn't," Peter says, entering the vault behind them. "Quite the elaborate scheme you have here Kate. Two countries, Aztec temples, Derek returned to a teenager. One that trusted you. One that loved you. All this complication just to gain access to our vault. Just to get your hands on that little piece of junk." He encourages her to turn it over so he can show her where they scratched off the Made in China. She accuses him of lying, but Peter claims that the truth is so much more fun. "...that little pendant is just a physical object to focus on." Talia used it to teach Laura and Peter tried to use it to teach Derek.
Outside, Scott roars and fights the Berserkers. Malia joins him. The Berserker throws Scott against a pillar and grips his head in its hand. The other one cuts Malia's legs and tosses her against the fence.
Kate grabs Derek and demands to know if the Triskelion is real. Derek is too distracted by Scott's roar and runs away to help him. Stiles and Lydia show up at the school. Lydia comments that Stiles needs to learn to use something other than a bat.
Malia scrambles away from the Berserker, and the second Berserker throws Scott over to join her. The two of them lay on the ground, not fighting. Kira runs in with her katana out and starts fighting them. She stops to look back at Scott, and he shouts her name in warning. She doesn't turn back in time and gets knocked down by the Berserker. She ends up on the ground next to Scott. Young Derek runs up behind the Berserkers and starts fighting them. He's got more advanced moves than Scott had and holds his own against two of them.
Peter tells Kate that Derek gave up on using the Triskelion as a focus when Peter taught him to use his anger as an anchor instead. "So you want to learn to control it? You want to get angry, Kate? Let's get angry!" Peter drama queens. They both bare their fangs, but someone else drops a bunch of wolfsbane bombs into the vault.
As Derek fights, he starts shifting back into his old self as he shifts into a wolf. Kate stumbles out of the vault. As Derek fights, there's a screeching call from someone that summons the Berserkers away. This is presumably Kate calling them off. Peter watches as a man comes into the vault and opens the safe as though it wasn't locked. He removes a suitcase and walks out, while Peter struggles to get up off the floor. Derek is returned to his old self. Scott says his name, and he turns around to look at him. But now his eyes glow yellow.
Lydia and Stiles run down into the vault to find Peter muttering to himself that it was never about the Triskelion. Lydia asks him what they took. "Bonds. Bearer bonds and they took them all." Stiles asks if that means they just got robbed. "This was a heist. Somebody planned this." "How much did they take?" "117..." "Thousand?" "Million."