Chris Argent remembers a fight that left many people slaughtered. The Oni were killing everyone, and the only reason he appears to have escaped is that he was underneath the dead body of a colleague. Scott is at the Argents' apartment, telling Chris about his encounter with the Oni at Derek's loft. Chris says there was something ritualistic about the way the Oni looked into his soul. Isaac agrees with that assessment. They say that the Oni go after anyone with a connection to the supernatural. Isaac asks who the guy was in Japan that Chris saw them go after. It was a yakuza boss, Chris's first gun deal. He was only 18. It was supposed to be a simple exchange, except Gerard didn't tell him that the buyers were yakuza. Gerard wanted to see him adapt to the moment and test his ability to improvize. The moment the sun went down, the Oni materialized. They had swords that weren't curved like katanas but were instead straight black steel, ninjatō. The Oni were after the Kumichō, or boss. The yakuza henchmen riddled the Oni with bullets, but that didn't stop their onslaught. Isaac asks if the Oni marked the Kumichō like they marked the rest of them. "Not exactly." The Oni gathered around the boss, who was some sort of supernatural creature, and impaled him with their swords. They lifted him off the ground, and the boss spewed blood. Chris doesn't know what sort of creature the boss was, but he thinks another survivor, a man named Katashi, might know. He's called Silverfinger because of unusual prosthetic. Katashi was going to take on the Oni himself, but Chris got up and shot one of them. He's known for a while that Katashi is in the country, "I spent yesterday tracking him down." Isaac points to the gash that Allison just finished stitching. "Didn't look like he wanted to be found." Scott asks if Katashi knows what the masked creatures are or what they want. Chris isn't sure. Isaac asks what he'll do if Katashi doesn't remember him. "He'll remember this," Chris replies and shows them the broken Oni mask. He says that he didn't kill it and he isn't sure that you can, but shooting off its mask did slow it down. "What was behind the mask?" Scott asks. "Darkness. Absolute darkness."
Kira sneaks into her room, trying not to make a sound. She lays down and remembers Scott bringing her home. He asks her if she's okay. "So... all of your friends are...?" "Yeah." "And you're a..." "Yeah." She touches his shoulder as she gets off the bike and then asks him if she can see him wolf out. He checks that the street is empty, then closes the visor on his helmet and shifts before taking the helmet off. Kira touches his face with wonder.
Scott is riding his bike to school, when the twins drive up on either side of him. Scott looks at them both and then pours on the speed. They speed after him. When they arrive at school and take their helmets off, Scott asks them if they're going to following him all day. "All day." "All night." Scott asks if it's about being in his pack, "It's about you being the target of demonic ninjas," Aiden says. Scott tells them that he doesn't need protection, but Ethan counters that they were looking right at him when the sun came up. Chris think the Oni are only able to come out at night, but Ethan says that since this is their first encounter with demonic ninjas they should play it safe. Scott resigns himself to the twins following him around. He tells them that he has to talk to Stiles without them around. Aiden objects, but Scott insists that that's how it's going to be. And the twins aren't going to listen in. "How would you even know?" "I'm a True Alpha, you have no idea what I can do."
Stiles and Scott go into the Chemistry lab, and Stiles relates what Caitlin told him about phosphors. "So that made me think of the chemistry closet and the fact that someone had to let Barrow in." Stiles heads toward the blackboard, expecting to show Scott the atomic numbers, but the board is blank. He then goes to show Scott that he still has the key to the closet, but the key is missing from his keyring. "I showed it to you, right? Didn't I show it to you?" Stiles asks Scott. "No, you just told me about it, I never actually saw it." Stiles starts to freak out a little. He tells Scott that he was there only a couple of hours ago and the message left to Barrow spelling out Kira's name was in his handwriting. Scott is confused about why Stiles would do the things he's claiming he did. Stiles says he knows how it sounds, but he pulls out a news report about Barrow and the bomb he used. "See this? See what he did? He put nuts, bolts, and screws. And then he hid the bomb and the detonator in a box that he wrapped as a birthday present. What does that sound like to you?" Scott agrees that it sounds like the prank they played on Coach. "That was my idea. You remember? That was my idea. That was no coincidence. It can't be." Scott isn't sure what he's supposed to say. He doesn't think Stiles is trying to kill people. Stiles crushes the story printout in his hand and shakes a little. Scott asks him if he's okay because he's looking really tired. Stiles admits that he hasn't been sleeping. Scott advises him to go home and take a sick day. Ethan and Aiden ask Scott if he told Stiles about the Oni. He didn't because Stiles has enough on his mind right now. Scott then suggests that maybe the Oni aren't actually after him. What if they're after Kira.
Back at the Argents' apartment, Chris says that their real problem is that Katashi doesn't see visitors. "He's a paranoid recluse who rarely steps outside the gates of his estate." Isaac asks how they're going to get to him, and Chris explains that Katashi has a thing for rare, antique weapons. Chris had some of his contacts put out the word that he was selling a piece from his collection. He takes out a French flintlock turnover pistol from 1645 housed in a plastic display case. Allison declares that her father isn't going alone. And Isaac adds that if Allison is going, so is he. Chris isn't terribly comfortable with this plan. Isaac believes that the Oni are going after Scott next because he's the one they were looking at when the sun came up. Chris says they're only going to have until nightfall to come up with a way to protect Scott.
Rafael McCall walks into the sheriff station on his cell phone. He says another week or two and they will be ready with their case against Sheriff Stilinski. "There's enough here to put Stilinski so far out of government work he won't be able to get a job riding the back of a garbage truck." McCall goes back into the evidence room, which he's using as an office, He opens up his laptop and sees a security alert. It's an image of Scott and Kira when they broke in to erase her phone.
Stiles is at the hospital talking to Mrs. McCall. She tells him that Dr. Gardener is not back until next week. She asks if he'd like to wait for one of the urgent care doctors. Stiles backs away from her desk. "Stiles, are you all right?" "I don't-- I dunno," he replies. He's on the edge of tears. Melissa takes him to a patient room and records his complaints. "Blackouts, but not for that long. Sleepwalking, which I used to do a lot as a kid. Also having some really bad anxiety." He's had a couple of panic attacks. "Oh, and I temporarily lost the ability to read, but that might've had more to do with this giant magic tree and the whole human sacrifice thing." Melissa asks him how many hours of sleep he's getting. He's gotten 8 in the last 3 days. She asks him if he's feeling irritable, and he says possibly to the point of homicide. "In ability to focus?" "No, the Adderall's not working." She asks if he's feeling impulsive, but he says it's hard to tell if it's worse than his usual. Then she asks about vivid dreams during the day, and he readily agrees. She walks over to him with a needle and says she thinks she knows what his problem is. She gives him a shot of medazolam, a sedative. "Why did you give me a sedative?" "Because you, Stiles, are one profoundly sleep-deprived young man. You need rest. You need it now. Lie down." She eases him back onto the bed. Stiles starts to ask how long it will take to start working, but he can already feel it. As Melissa pulls the covers up over him, he holds her hand. She brushes his hair and tells him to get some rest. As he drifts off, he says, "Thanks, mom."
Chris, Allison, and Isaac are sitting in the car outside Katashi's estate. Chris gets a text telling him that Katashi won't do the exchange in person. "He's a paranoid recluse, shouldn't you be a little less surprised?" Isaac asks. Chris replies that he was trying to remain optimistic that they wouldn't have to go to his plan B. When Isaac asks him what plan B is, Chris asks him how tall he is.
It's the end of the school day, and Scott is ready to head home. He tells Ethan and Aiden that while he appreciates them wanting to protect him, he has to be alone for a bit. He apologizes to them and drives off, while the twins are unable to start their bikes. Scott picks up Kira, who has the spark plugs from the motorcycles. They drop the plugs on the ground and speed away.
Chris shows Allison a blueprint of Katashi's estate. Isaac calls over to them and says that this isn't going to work because he looks ridiculous. "I look like I just stepped out of the last period of a Catholic prep school. and there is no way that i am going to be able to remember all this, I mean, what does this even mean? Revolving over and under barrels?" Allison and Chris exchange a look. Chris says he just has to keep Katashi's men talking long enough for them to infiltrate the building. Allison assures Isaac that he looks great, but he complains that he's sweating through his jacket and didn't know that he could sweat that much. Allison asks her father to give them a second. She pulls Isaac around to the back of the car and assures him that he can do this. "You're not a boy if you walk in there acting like a man, Go in there with confidence and all they'll see is a boyish-looking man." Isaac continues to doubt himself, so Allison kisses him. She moves his hand down to her ass and then asks him how he feels now. Isaac walks into the estate wearing sunglasses and a smirk. "Gentlemen, I believe Mr. Katashi's expecting me." A swarthy werewolf named Kincaid with a deep voice replies, "I'm expecting you."
Scott and Kira enter Scott's house, and he starts locking all the windows and doors. Kira voices concern that they're running out of time. Scott tells her that Allison, Chris, and Isaac are working on something that can help. Kira is doubtful that closing the windows is really going to keep them out, but Scott explains that they've installed a security system because a lot of things happened a few weeks ago. Scott's boss helped make some changes. "Your boss. Do you work for a security company or something?" "No, he's a veterinarian." Scott explains that he can't arm the system, only his mom can. They set up mountain ash all through the house, and only a human can close the seal. Scott goes up to his room to close his window, and when he backs up, he bumps into Kira. He apologizes and holds her upper arms for a little longer than necessary. Kira doesn't seem to mind. Scott assures her that they're going to be okay. She's certain that the Oni are really after her, not Scott. She assumes that he didn't bring the twins to the house because he didn't want them to get hurt. "I don't want anyone to get hurt," he replies. Kira asks why he brought her then. She quite reasonably asks if she shouldn't be somewhere else rather than between Scott and the Oni. He insists that he's not going to let her get hurt. "But I don't want you getting hurt, especially because of me," Kira says. Scott replies that it could be either of them that the Oni are after. Kira is still determined that they're after her. She takes out a children's book of Japanese mythology and shows him the story of the kitsune.
Melissa checks in on Stiles, who is still sleeping peacefully. She looks at his chart and realizes that his symptoms look familiar. She goes to the records room and gets Claudia Stilinski's file. The files says: Patient is irritable and impulsive. Complaints of vivid dreams during the day, inability to distinguish between fantasy and reality, occasional hallucinations. Symptoms began with acute onset insomnia.
Back at Katashi's estate, Chris and Allison take out some guards, while Isaac shows the buyers the gun. "What we're looking at here is a mint condition French flintlock turnover pistol. Crafted in the mid-seventeenth century, it was a gift from Louis XIV to a prominent French family. It's only ever been fired once." The werewolf adds that it was fired during a duel between brothers on the grounds of the Palace of Versailles. Isaac is a little thrown that they're familiar with the gun's story, and the werewolf replies that Mr. Katashi knows what he wants. He opens up a case with $150k, which Isaac says he will have to count. Isaac pulls out a money counting machine and starts putting the stacks of cash through.
Kira sits on Scott's bed and shows him the kitsune myth. She says that when a kitsune rubs its tails together it can create fire or lightning. This is called fox fire. Kira quickly tells Scott that she doesn't have any tails, and he asks how she knew what he was thinking. "Because kitsune are also psychic. I can hear all of your thoughts." Scott looks concerned for a second until Kira tells him that she's kidding. She notices that the sun is setting. Scott asks if she's scared. "Not right now." She says that she has to head home for dinner, and Scott suggests telling her parents that she can't make it. "Say you're going on a date." Kira smiles at him. "Are you sure that's a good idea. You know foxes and wolves don't really get along." She turns the page in her book to show him, but Scott closes the book completely and almost kisses her. They stop because Scott hears a car pull up. He thinks it's his mom, but it's his dad. "What are you doing here? And why do you still have a key?" Agent McCall tells him that it's funny that he mentions keys because while he has a key to the house, he's not sure how Scott got a key to his office. He turns his laptop around and shows them the photo it took of them when they broke in.
Isaac is about to go for another wad of cash when the werewolf slams the briefcase shut. "You don't know the whole story behind this gun, do you?" He explains that there was no duel between brothers. That was just a cover story for an encounter in the woods with a vicious animal. One of the brothers got bitten. And based on the Code, the one brother killed the other who had been turned with the pistol. Isaac tells him that that's better than the version that he heard and announces that he's going to trust them that the money is all there. He goes to leave, but the werewolf grabs his arm. Isaac partially shifts and growls at the werewolf guard. Mr. Katashi is in his office watching the showdown with Isaac via video camera. He draws a gun and points it at the door, where Chris soon appears. "All I want to do is talk." Chris orders Allison to disarm Mr. Katashi as she enters the room from the other side. Allison uses a chain whip to grab the gun out of his hand. Chris asks again if they can talk. The werewolf guard brings Isaac into the room with his claws at Isaac's throat.
Scott doesn't have a ready explanation for his father. Agent McCall says, "This kind of thing usually begins with something along the lines of 'It's not what you think' or 'I can explain.'" Scott looks out the window at the dying light and then tells his father that he needs to leave. Agent McCall isn't going without a satisfactory explanation. Scott tells him to get a warrant. "I don't need a warrant, I'm your father." "No, you're a gene donor. I got my hair color from you, and that's all I got. So you're not allowed to play Tough Dad with me." Melissa comes in, then, and asks what's going on. Agent McCall leaves it to Scott to explain, but the shadows fall outside, and an Oni appears in the living room. Agent McCall goes to confront the Oni, and it stabs him in the shoulder. Melissa pulls Rafael out of the way, as Derek opens the door to Scott's house and comes sliding in. Scott and Derek fight the Oni, and Scott tells his mom to seal the mountain ash circle around the house. An Oni appears behind Kira. The twins burst through a window to take it on. They all concentrate on throwing the Oni outside the house. Derek throws the last Oni out, and Melissa runs to the front door and smashes a bottle of mountain ash on the threshold. It forms a perfect line and closes the seal. Kira and Aiden stand by the door, watching the Oni. She attempts to reach out beyond the door but is repelled by the seal.
Chris unwraps the broken Oni mask for Mr. Katashi. "Starting to remember? It was 24 years ago." Katashi signals to his man to let Isaac go. Katashi tells them that the masked creatures are called Oni. "They are demons and they are unstoppable."
Derek asks Scott if all the baseboards are ash wood. "Yeah, it was Deaton's idea. Where the hell did you come from?" "Been following you." "For how long?" "All day." Derek says this as though it's obvious. Melissa is trying to treat Rafael's wound. She tells Scott that it isn't good. "Need to call for backup," Rafael says. Melissa tells Scott that from the way his arm is rotated, the tendon looks torn and he could be on his way to a collapsed lung. Scott tells her that the Oni aren't going to leave until the sun's up. "At the rate he's bleeding, he's not going to make it that long. We gotta get him to the hospital." Scott thinks maybe they should call the Sheriff. As a human, he would be able to pass over the mountain ash threshold. Melissa asks if calling him is just going to get more people hurt. It might, so they don't make the call.
Katashi tells Chris that nothing can hurt the Oni. "At least no man-made weapon. The Oni are a force of nature. You don't fight a tsunami, you endure it." Allison asks him how you endure it, and Katashi tells her that one of them already has. He turns Isaac's head and explains that the mark is the Japanese kanji for "self." The mark means that Isaac is still himself. "The Oni are looking for one who is no longer himself." They are looking for someone possessed by a dark spirit.
Aiden asks Kira what she is, and she pretends that she doesn't understand. Aiden takes her hand over Ethan's objection and brings it to the barrier so his brother can see that she can't pass through. "So what are you?" "She's a kitsune, idiot," Derek says. "Use your eyes. You can see it all around her. The younger ones give off an aura. She just hasn't learned how to conceal it yet. She probably doesn't know what kind she is, either."
Katashi explains that there are 13 kinds of kitsune. Among them, celestial, wild, ocean, thunder. "But there is one a dark kitsune. They call it void or nogitsune." Chris realizes that the Kumichō he saw the Oni kill was a nogitsune. Being possessed helped him rise through the ranks of the yakuza. The nogitsune draws its power from pain and tragedy, strife and chaos.
Back at the McCall's house, Scott insists to Kira that she can't be what the Oni are after because her aura doesn't look evil. She argues that that's what a shapeshifter and trickster would do: not appear evil. "How do you know I'm not the bad guy?" Kira asks him. "I've seen the bad guys. And you're not one of them." Aiden tells Derek that he wasn't going to hurt Kira. "Not yet," Derek replies, not looking at either twin. The twins explain to him that they're there to protect Scott and fight for him. "I'm sure you are. I'm sure you'd kill for him. But are you willing to die for him?" The twins look at each other and don't answer.
Katashi asks Chris if he knows why he cut off his finger. "Penance," Chris replies. Katashi explains that there's a ritual called yubitsume, removing the joints of the little finger weakens the grip on the sword. When a katana is gripped properly, the little finger is the strongest. Isaac asks why they took his finger. "They didn't take it. The yakuza performs the ritual himself and offers it to his superior." It was penance for the fact that he was going to run from the Oni. "That shot you fired saved me from looking like a coward before the survivors." If he had been so humiliated, he would have lost his head. Katashi wraps the Oni mask back up and pushes it across the table to Chris. "I wish I could give you the answer you need, Mr. Argent. I owe you more than my life. I owe you my honor." Katashi advises that if there is a nogitsune among them, they should let the Oni destroy it, even if it's his own daughter.
The Oni outside Scott's house start beating on the barrier, shaking the whole place. Dishes start to fall and break as they pound on the barrier. The Oni then start pushing against the barrier continuously, much like Scott did when he burst through Jennifer Blake's mountain ash force field. "Guys," Ethan says. "We have a problem." Agent McCall tells Melissa that he needs to speak to Scott. His reason for coming back to Beacon Hills has something to do with what he wants to say to him. Allison calls Scott and advises him to stop fighting the Oni and just let them do their test. Kira asks Scott if the Oni are looking for her. "They're looking for a dark spirit and I know it's not you." Derek warns Scott that they're about to break through. The Oni step inside, and Scott tells everyone not to do anything. Aiden can't believe what he's hearing. Scott takes Kira's hand, and they submit themselves to the Oni's test. Derek holds Aiden back from intervening. The Oni toss Scott and Kira aside, giving them both the mark. Derek and the twins rush to help them.
Stiles wakes up in the hospital alone and starts wandering around. They rush Agent McCall into the ER. "Stab wound to the left chest, possible collapsed lung, and pulse is weak." Scott goes to find Stiles. Stiles walks into a dark, empty room. The Oni appear around him. They go to grab his head to perform the test, and suddenly Stiles grabs the Oni's hand. He looks, for a moment, surprised that he did it. Then he glares at the Oni and bends its arm away. Stiles shoves his other hand into the Oni's chest and pulls out a firefly. The Oni vanishes. Stiles turns toward the two Oni behind him. They rush him, and the scene cuts away. Scott finds Stiles standing in the room alone. "Stiles. You okay?" At the sound of Scott's voice, the possession seems to drop, and Stiles turns around looking innocent. "Yeah. Fine. What's been going on?" Stiles pats Scott on the chest, and they leave. A firefly dies on the floor.