Lydia is in the shower, and suddenly gets the sense of being watched. She shuts off the water and slowly pulls back the curtain to reveal the lacrosse field and a stand full of fans. She's in the middle of the field in her winter formal dress, and everyone is cheering for her. Then she turns and sees Peter Hale stalking out of the dark. She tries to run, but he grabs her, throws, her to the ground, and crawls on top of her. She wakes up from her nightmare with a shout. Her sheets are covered with dirt that she's carried in from a night of acting without her knowledge. "Leave me alone," she says. Peter is in bed next to her, and he sighs a little, telling her that he can't, not yet. She asks if he's real, and he tells her that the answer is also "not yet." He says that everything will get back to normal once she does everything that he asks. Peter leads her down the stairs and explains to her that everything needs to happen on the next full moon. He explains that the full moon of March is called the Worm Moon because it's the last full moon of winter and the worms start to crawl out of the earth. Lydia points out that the full moon is on Wednesday, which is also her birthday. Peter says that her birthday is always the party of the year. He tells her that they're going to make it a very special party. She asks what will happen if she doesn't go along, and he shows her a vision of her party guests murdered. "Why me?" "Because Lydia Martin is not only beautiful, not only incredibly intelligent, she's immune." She asks what it is that she's immune to, and Peter realizes that no one has told her about Werewolf yet. He says that it's probably best if he just shows her, and then he appears in his alpha form outside her living room window. Lydia screams, and Peter breaks through the window to leap at her.
The Argents are back home, and Chris frantically cleans out Victoria's bite. He tells her that it's not that deep, hoping it will mean that she isn't going to turn. Gerard comes down the steps to the basement to watch them. Victoria puts her hand on Chris's and tells him that it's all right. Chris tries to argue with Gerard that they can't know if the bite is enough to turn her. Gerard responds that he's the cold-hearted patriarch holding the family to its commitments. Chris asks how he's supposed to get Allison through losing her mother when they just buried her aunt. Gerard asks him when he thinks Allison would ever be prepared to handle the death of her mother. Gerard tells him that "That thing over there is just a cocoon waiting to hatch."
Derek and Scott return to the subway tunnel, and Derek explains that they need a new plan because next time they might not be able to heal. Scott understands that they can't save Jackson; Derek replies that they can't seem to kill him either. "I've seen a lot of things, Scott. I've never seen anything like this. Every new moon is just going to make him stronger." Derek admits that he doesn't know how they're going to stop the kanima, if it's even possible. Scott suggests letting the Argents handle it, but Derek feels responsible because he turned Jackson. "Yeah, but you didn't turn him into this. This happened because of something in his past, right?" Derek says it's just a legend in a book and nothing is ever that simple. Scott asks what Derek's keeping from him, and Derek is insulted that Scott always thinks he's hiding something. When Scott responds that that's because Derek always is, Derek says that he does it to protect him. "Doesn't being part of your pack mean no more secrets?" Scott asks. Derek dodges this manipulation and tells him to go home, sleep, heal, and check on his friends. There's a full moon coming, and Derek's sure it's going to be a rough one.
Allison drops Matt off at home. He apologizes for kissing her, and she tells him not to worry about it. He asks her if she and Scott are still together. "Not really." "Not really. God, I hate not reallys. You never know what to do with not really." She asks if he'd understand if she said it was complicated, and he replies, "Not really." They both laugh. He touches her hand and then gets out of the car. After he's gone, Allison breathes on the window to check for a message from Scott, but there isn't one. She then realizes that Matt left his bag and camera in her car and inspects the camera. The first few photos are of the lacrosse team and make her smile. But then she sees that Matt has a lot of candids of her. There are even photos of her at home in her bed reading. Allison realizes that Matt's been stalking her. Matt appears at the car door, and Allison quickly puts the camera down. He tries to open the door, but it's locked, so he silently asks her to open the window. She does, and he says that he forgot his bag. Then he asks her if she liked the photos, and she tells him that the lacrosse ones are really good. He says there's a good candid one of her and he could show her some of them on his computer if she wanted. She tries to put him off to another night, but he asks for a few minutes. She tries, "It's getting kind of late" but Matt just laughs and tells her it's the weekend. He also claims that it's spring break and asks if she has anything going on on Saturday. She again tells him that she wants to leave, and this time he leans back out of the window and lets her go.
On Wednesday night in the subway station, Derek opens his box of chains, and Isaac notes the triskele stamped on the lid. Boyd says, "The spirals mean different things. Past, present, future. Mother, father, child." "You know what it means to me?" Derek asks. "Alpha, beta, omega?" Boyd guesses, and Derek seems impressed that he figured it out. He explains that it means they can all rise to one or fall to another. Betas can become alphas, but alphas can also fall to betas or even omegas. Isaac asks about Scott being an omega, and Derek declares that Scott is part of the pack. Isaac is skeptical and asks where he is if that's the case. Derek explains that Scott is out looking for Jackson. Derek tells them that the price they pay for their power is bloodlust on the full moon. Erica quips that it's a good thing she had her period last week, then. Derek smirks and holds up a crown of spikes that she will get to wear.
Lydia arrives at Allison's with Macy's bags full of clothes for her to try on for the party. Lydia is planning to wear at least 3 outfits for the evening. Allison notes that Lydia didn't send out any invites, and Lydia explains that her party is the biggest event of the year, so there's no need for invites. Allison's concerned because things have been off lately. She brings up Jackson, and Lydia stares at her. Allison asks if Jackson's coming to the party, and Lydia replies that everyone is coming to the party. Lydia continues to pick through her purchases when Victoria comes in to ask Allison if they can talk. Allison puts her off, and Victoria tells her that sooner would be better than later. Allison doesn't give her a solid reply, and Victoria leaves without being sure they will have a chance to talk privately.
Stiles looks through the Beacon Hills 2006 yearbook again. He's crossed off all the people that have been murdered so far. His dad stops to ask him what he's up to, and he absently answers homework. The Sheriff comes back to the door when he remembers that it's spring break. He comes into the room and closes the yearbook. The Sheriff tells him that Mr. Harris was brought in for questioning and they're working on a warrant to arrest him for the murders. Stiles asks what proof they have, and his dad says they have tire tracks near Sean's trailer that match Harris's car. Someone also saw the car outside the hospital where Jessica was killed. He tells Stiles about the bumper sticker, and Stiles tells him that he saw the car outside the rave. Stiles will have to give a statement, since that makes him a witness. Stiles is concerned that Kara wasn't in Harris's class and that Mr. Lahey doesn't fit the pattern, but his father says that being at the sites of three murders is damning evidence. Stiles still doesn't feel it's enough evidence and goes back to looking through the yearbook. His dad thought he hated Mr. Harris. "I don't hate him. He hates me," Stiles says, but he still feels there's something missing. The Sheriff tells him that he doesn't have to solve the case for him, but Stiles thinks that he does. Just then, the Sheriff looks down at the page Stiles has stopped on. It's a photo of the swim team. All of the victims were on the team, and Mr. Lahey was the coach.
Lydia finds Jackson at the school coming out of the boys locker room. She asks him if he's coming to the party, and at first he's zoned out. When she touches him, he comes to and tells her that she doesn't want him there. She attemps to tell him that she's over the breakup, but Jackson grabs her arm and insists that she doesn't want him there. She doesn't take no for an answer, even though she backs away from him slowly.
At 10pm, Allison approaches Lydia's house. A couple of people whoop and rush ahead of her to the party. Stiles rings the doorbell, and when Lydia opens it, he's carrying a box too big to fit through the door. "Happy birthday!" he sings and shakes the box. She is clearly stunned. Stiles attempts to the shove the box through the door, and Lydia turns on her heel and leaves. Inside, Stiles and Scott quiz each other on whether they've seen Jackson or Allison. Stiles says they need to tell Allison what they found, but Scott isn't sure what it is that they've found. "We figured out it has something to do with water. The fact that all the victims were on the swim team. The way the kanima reacted around the pool." The kanima's master really hated the 2006 swim team, Stiles concludes, and thinks it could be a teacher or another student. He wonders what they haven't thought of. They meet up with Allison by the pool, and she reports that Jackson isn't there. "Yeah, no one's here," Stiles replies. Scott kindly suggests that they all could be early, but Stiles thinks it's because Lydia now has a reputation as the town whack job. Allison says they have to do something because they've ignored her for the past two weeks. Scott is oddly defensive of Stiles, claiming that Lydia has ignored him for 10 years so they don't owe her a party. Allison thinks that Lydia wouldn't be the town whack job if it weren't for them. Scott offers to get the lacrosse team to the party. Stiles claims he has some people he can call who know how to party. He calls the drag queens he met at the Jungle, and they show up with their friends. Lydia gives them a once over and then invites them in.
Meanwhile, Derek chains Isaac, Boyd, and Erica up in the subway car. Isaac asks why Erica has to wear the headband, and Derek explains that Erica will be able to withstand more pain than either of the boys. Isaac holds her while Derek screws the bolts into her head. Boyd looks really upset.
Lydia's party is in full swing. Stiles asks Scott if he's going to apologize to Allison. Scott wonders why he should. "Cause you're the guy. It's like what we do." "But I didn't do anything wrong." "Well you should definitely apologize. Anytime a guy thinks he hasn't done anything wrong it means he's definitely done something wrong." Scott declares that he isn't apologize, and Stiles asks him if that's the full moon talking. Scott agrees that it probably is and wants to know why Stiles cares. Stiles just wants to see something go right, because they're getting their asses handed to them. "People are dying. I got my dad fired. You're going to be held back in school. I'm in love with a nutjob. And if on top of all that I gotta watch you lose Allison to a stalker like Matt, I'm gonna stab myself in the face." Scott tells Stiles not to stab himself in the face because Jackson just arrived at the party. Lydia turns around and is surprised and pleased to see him. She gives him a glass of punch. After Jackson takes a sip, we see that the punch bowl has wolfsbane floating in it.
In the subway car, the betas all scream and pull at their chains. Isaac asks Derek how he can't feel the bloodlust. "I feel every second of it." "Then how do you control it?" "Find an anchor. Something meaningful to you. Bind yourself to it, keep the human side in control." Isaac asks what Derek's anchor is, and he says that it's anger, but it doesn't have to be that for everyone. Derek finishes chaining Isaac up, but Isaac pulls on the chain and the seat leg comes loose. Derek looks worried.
Chris opens up a case of drugs and needles. Victoria is insulted that he thought she would commit suicide with prescription drugs. He tells her that gender statistics most women-- She picks up a kitchen knife instead. "But you're not most women," Chris says. Victoria goes upstairs to write a letter to Allison. Chris sits down, and Gerard advises him to not hesitate because of Allison. She will feel the ground shift beneath her feet many times during her life, so she'd better get used to it. Gerard tells Chris to go help Victoria die with dignity.
Lydia has switched dresses. She keeps handing out more punch. She gives a glass to Matt, and then Allison and Matt see each other. She ignores him, and Matt tosses out his drink. Scott inspects his hand, presumably making sure it isn't turning into a claw, and Lydia slaps a drink in it. Scott tells her that he isn't drinking tonight, and she asks what's up with the two of them. She means Scott and Allison. She thinks her party might be the perfect time for them to have a good time. Scott agrees and takes a sip of punch. When he looks over again, Allison is gone. 2Matt asks Allison for a chance to talk, so she gives him two minutes. He tries to close the door to the bedroom they're in, but she tells him no. He explains that he took some pictures that he probably should have told her about but asks if it's that bad that he thinks she's beautiful. She tells him that he's being a stalker. He gets angry that she thinks his bedroom is wallpapered with photos of her. You think I'm the kind of guy who's gonna say something like, "Well if I can't have her, no one can"? He says that another pretty girl will walk through the room in five minutes, so she says he's in luck that he'll only have to wait another three. When she tries to leave, he grabs her arm, and she throws him to the ground on instinct. Allison sees someone walking through the party with a crossbow. She thinks it's a hunter and starts calling Scott's name to warn him. The figure shoots her in the stomach with a bolt instead. When she looks up, she sees herself, disgusted with how she's always yelling for help. The vision Allison raises the crossbow again and fires, and then the hallucination is gone. 4Stiles asks Scott if he's feeling okay. Scott says it's not the moon but something else is wrong. Stiles then hears his father's voice. "Why am I wearing black? What are you, an idiot? I just came from a funeral, you know people wear black at funerals." He's yelling at some kid at the party and then shoves the kid away. Stiles is struck dumb. The Sheriff then turns to him. "It's you. It's all you. You know every day I saw her lying in that hospital slowly dying. I thought how the hell am I supposed to raise this stupid kid on my own. This hyperactive little bastard who keeps ruining my life. It's all you. It's you, Stiles. You killed your mother, you hear me? You killed her. And now you're killing me." The Sheriff then hurls his whiskey bottle at Stiles, and the hallucination ends before it can hit.
Victoria sits in Allison's room looking at some photographs. She says she never got a chance to talk to her before Allison left for the party. She wants to kill herself in Allison's room so she can feel close to her, but she's going to need Chris's help.
Everyone at the party is acting more drunk and drugged. Scott sees Jackson kissing Allison on the landing. Then Jackson changes into the kanima, and Scott starts to wolf out. When the hallucination ends, Scott goes to find Lydia. Jackson snaps out of his trance suddenly and drops his drink. He sees the backs of two people and hears them say that they're looking for him. The people ask Lydia to tell Jackson that his real parents are there to see him. They turn around, and their faces are blank. Then Jackson's face goes blank as well. Scott comes through the crowd looking for Lydia, but she's gone. She's left the party.
Isaac, Boyd, and Erica all shift and strain against the chains. Derek calls Scott and leaves him a message that he's probably going to need some help with the betas. The betas start breaking out, and Derek whips out his claws. Erica is the first one to get free, and she takes a swipe at Derek. While Derek is occupied, Isaac leaps out the subway car window, and then Boyd attacks from behind. Erica and Boyd get Derek pinned to the ground and attack him.
Scott tries to get Stiles to drink some water so he can sober up. Danielle comes up next to him and asks him what he's doing. She offers to help and dunks Stiles's head in the pool.
Derek manages to chain Erica back up to one of the poles in the subway car. Isaac comes back in and attacks Boyd, but this time he's under control and doing it to help Derek. They nod at one another in recognition.
Victoria holds the knife to her chest and tries to shove it in. She says she can feel the change happening. She makes Chris promise to tell people that she had a history of depression. He promises. Allison will have to keep up the lie too, but Chris won't let her believe it. He promises that Allison will know it was the hardest thing she ever did and that she did it for her family not to them. She holds the knife to her chest but she can't do it herself and asks him to help her. She leans back into his arms, and they both hold onto the knife together. Her eyes go gold, and she tells him to do it. They shove the knife in, and Chris cries.
Derek chains Isaac back up, even though Isaac is under control. He tells Derek that he found an anchor: his father. Derek is confused by that. "Your father locked you in a freezer in the basement to punish you." "He didn't used to." Derek leaves the subway car and turns to find Lydia standing there. He just stares at her, and she blows purple powder into his face. It knocks him out.
Allison arrives at the hospital crying. She sees her father, and tells him that if this is a sick training session-- But she can tell by his expression that it isn't. She cries and demands to know what happened, but he just hugs her and apologizes.
At the party, Stiles and Scott don't know what to do because everyone is freaking out. Someone throws Matt into the pool, and he starts screaming that he can't swim. Jackson pulls him out of the water. Matt glares at everyone, but especially Scott and Stiles. The cops arrive and everyone takes off. In the chaos, Scott sees Matt outside with the kanima right next to him. They disappear into the crowd.
Lydia somehow dragged Derek from the subway lair into her car and then into the Hale House. He's starting to come to as she drags him across the floor. She has turn up the floorboards over where Peter is buried. Derek begs her to stop. She puts his arm in Peter's hand. "You don't know what you're doing," he breathes. She then adjusts some mirrors she has setup around the room and bounces moonlight directly down onto Peter. Peter's claws dig into Derek's arm, and his blood brings Peter back to life. Peter bursts from the ground, and Derek tries to get away. "I heard there was a party. Don't worry, I invited myself," Peter says.