As Jackson and Danny lift weights in the school weight room, Jackson forces himself to lift weights that are too heavy, and Danny seems somewhat concerned, though he ultimately becomes frustrated by Jackson's refusal to allow him to spot him and goes to take a shower, adding that he gets to inherit Jackson's Porsche if he gets pinned below the weight and dies. Just as Jackson becomes awestruck at his sudden ability to hear everything in the school, Erica shows up and takes Jackson to Derek, who is waiting in the railway depot, the Hale Pack's lair, with Boyd and Isaac.
Derek first attempts to interrogate Jackson for information about the Kanima and Jackson's relationship to it, but when he's unable to give him the answers he wants, Derek decides to test Jackson by forcefully putting several drops of Kanima venom into Jackson's mouth, believing that "a snake can't be poisoned by its own venom." However, Jackson is paralyzed by the venom, leaving Derek to continue the search to find out who the Kanima is. While Jackson is immobilized and vulnerable, Isaac forcefully demands that Jackson takes back his testimony regarding Mr. Lahey's abuse of Isaac and their fight the night of his father's death so that he is no longer a fugitive, a demand with which Jackson reluctantly complies; Sheriff Stilinski is appalled to find out that Jackson lied, both because of the harm it caused Isaac as well as the fact that it adds more reason for his superiors to think he is unfit for his job.
Back in school, Jackson listens to a conversation that Erica and Isaac are having in the hallway, where he learns that they want to use more of the Kanima venom that Derek found to test Lydia during chemistry class; if she fails the test, they plan to kill her on Derek's orders to stop the Kanima's reign of terror. When Jackson brings this up to Scott and Stiles in class, however, Stiles, disagrees with Isaac and Erica's theory, stating that while he saw pure evil in the eyes of the Kanima, he only sees "60% evil" in Lydia's eyes.
Meanwhile, during this conversation, Lydia begins hallucinating while she's up at the chalkboard doing a problem. In this hallucination, she sees Peter Hale drawing spirals on the chalkboard before then pushing her against the wall, crushing a piece of chalk in his hand, and blowing the powder in her face. When she finally snaps out of it, she (as well as Jackson, Scott, and Stiles) are horrified to see that she has written "SOMEBODY HELP ME" backwards numerous times on the chalkboard.
The group then goes to chemistry class, where Erica and Isaac plan to follow through on their plan to test Lydia, and Allison, Scott and Stiles plan to protect her from the Hale Pack. Mr. Harris' experiment for the day's class involves making rock candy out of sugar and water, with the class rotating in pairs. Scott and Erica are paired up first, where Scott begs her to give him a chance to talk to Derek before they do anything. Erica changes the subject to his relationship with Allison, implying that she knows they are not as broken up as they pretend to be before she reminds him that if he was with her, he would be able to be with her at all time instead of behind his and Allison's parents' backs.
Allison is first paired with Lydia, who is surprised when Allison assures her that she isn't jealous of Erica's attempts to seduce Scott. Stiles is first paired up with Isaac, who he threatens to "turn his little Werewolf ass" into a fur coat as a birthday present for Lydia if Isaac so much as harms "one perfect strawberry blonde hair on her head." Isaac recounts when Lydia viciously turned him down after he asked her out freshman year and that he planned to channel his negative energy into killing her.
After the couples are rotated, Allison is paired with Erica; when Allison demands to know what she plans to do to Lydia, Erica counters that the better question is what Lydia planned to do to them before remarking that she has always been a little "psychic" and that she thinks Allison and Scott aren't likely going to last. Allison asks her if she thinks she can hurt her just by sliding her hand up Scott's thigh, and Erica retorts by asking if she'd prefer it was hers before suggesting that if they got into a "girl fight" in lab would be hot. Just then, Harris informed them that, if their test went properly, they should have a sugar crystal that they can eat. Scott and Stiles look up to find that Isaac, who was paired with Lydia after the rotation, has just handed her the sugar crystal, which has been coated in Kanima venom. Scott tries to stop Lydia before she eats it, but he's too late-- Lydia licks the venom-coated crystal and is completely unaffected by it, much to Allison, Scott and Stiles' horror and Isaac and Erica's satisfaction.
Elsewhere, Danny and Matt are trying to help Jackson figure out who edited out the two hours of video he made of himself during the full moon. Their theory is that somebody who has a key to Jackson's house could have gotten in and edited out the video footage. Jackson realizes that he gave Lydia a key to his house, and that she still has it, leading him to suspect that it was Lydia who manipulated the footage. After class, Allison goes to Marin Morrell, the school guidance counselor and French teacher, to translate a page of the Argent Bestiary, specifically the part about the Kanima. Ms. Morrell translates it, though she admits that her skills at translating Archaic Latin are not perfect, and claims that the Kanima is like a werewolf, but instead of seeking a pack like a Werewolf, the Kanima seeks a friend. Meanwhile, as Danny continues to try to recover the lost footage, Matt, who is working on his own photography on his laptop, discovers that all of the photos he took of Scott at the most recent lacrosse game caused a strong lens flare off of his eyes.
Out on the lacrosse field, Scott approaches Boyd and attempts to talk him down from the plan to kill Lydia, to the point where they briefly fight until Derek arrives and intervenes. Derek continues to try to convince Scott that killing Lydia is the only way to ensure that no one else is killed by the Kanima, but Scott argues that Lydia is different and that Derek's test didn't prove anything. Derek maintains that Lydia is the Kanima by bringing up the fact that Lydia was bitten by Peter when he was still an Alpha, even after Scott points out that the Kanima is not like a Werewolf. Derek retorts that Kanimas and Werewolves are both shapeshifters and informs him that, in the supernatural world, sometimes the shape one takes after being turned reflects the person that they are; since even Stiles, who has had a crush on Lydia since third grade, has referred to Lydia as "cold-blooded," he thinks this is more than enough proof that Lydia is the Kanima.
Scott goes on to ask if Lydia could be immune to the Bite and the venom, an argument which begins to cause Derek to question himself, though he refuses to allow Scott to see or think that he is doing so; instead, Derek exasperatedly insists that he has never seen it happen, so it couldn't be true. Seeing that Derek's argument is weakening, Scott correctly guesses that Derek gave Jackson the Bite in hopes that he would die rather than be turned, but since neither seems to have happened (and Derek seems to have no idea why it happened), Scott shares his theory-- Lydia is immune to the supernatural and somehow passed it on to Jackson while they were still in a relationship. Derek stubbornly refuses to believe this and makes it clear that his mind is made up, leading Scott to admit that while he was hoping he could convince him not to hurt Lydia, he wasn't counting on his ability to do so.
Back inside the school, Erica and Isaac (the latter of whom is sharpening his claws by running them along the row of lockers in the entrance hall) head into the classroom, only to find that Allison, Scott, Stiles, Jackson, and Lydia are nowhere to be found. The group is seen rushing through the parking lot toward Stiles' Jeep, where they have convinced Lydia that they are holding a study group at the McCall House, where they will be joined by Scott later on. When Lydia becomes suspicious of her friends' strange behavior, including locking all of the doors and windows in the house, Stiles lies and states that there have been a number of break-ins and murders in the neighborhood.
Upstairs in Scott's room, Jackson confronts Lydia about the key to his house that he gave her, and Lydia, hurt by Jackson's increasingly hostile behavior toward her ever since their breakup and his demand to get his key back, rants, "Are you kidding? I'm attacked by some lunatic who bites me-- a lunatic who, by the way, still hasn't been caught. I spent two days freaked out of my mind, walking around the woods naked, all of my friends are acting like total nutcases, and you expect me to be worried about some stupid key?" Jackson angrily demands to know if she has it or not, and Lydia stands her ground by saying that she doesn't have it, though Jackson's enhanced senses allow him to determine that she's lying.
Jackson then accuses her of editing out the footage of his transformation on his first full moon, stating that she was either trying to take his eagerly-awaited moment or was trying to protect him, though Lydia has no idea what he's talking about and says as much. She then takes his key off of a chain around her neck and gives it back to him before angrily and tearfully insisting that she hates him. Jackson is hurt by these words and begs her to say that she actually loves him, and though she refuses to say so, the two begin making out. Unbeknownst to Lydia, however, Jackson has begun transforming into his Kanima form with scales developing on his neck and back.
Downstairs, Allison and Stiles are looking out the window and are horrified when they see the full Hale Pack, Derek, Isaac, Erica and Boyd, standing across the street and staring at the McCall House, causing Allison to frantically call Scott and urge him to come home as soon as possible. After the call, Allison gets out her mini-crossbow and grimly remarks that she may have to call her dad, though Stiles is clearly not thrilled about the prospect, especially considering that it would be revealing that Allison and Scott are still seeing each other behind their parents' backs.
Stiles suddenly gets the idea to have Allison shoot one of them, since they told Scott they could protect themselves and they should give it a shot. When Allison asks which person, Stiles immediately tells her to shoot Derek, preferably in the head. Allison, distracted by the conversation, reminds him that since Scott could catch her arrow in mid-air, Derek likely could, too, leading Stiles to exasperatedly instruct her to shoot one of the new Betas. Unfortunately for them, when they looked back out the window, they find that Isaac is missing.
Allison and Stiles are shocked when they come face to face with Isaac in the house and are in the midst of panicking when suddenly, Scott finally arrives and incapacitates him. Upstairs, Lydia, who has just been told that someone is trying to break into the house, attempts to call 911 just as Allison is confronted upstairs in Scott's bedroom by Erica, who admits that she's always been curious about the "sick rush of power" one can get by stealing another girl's boyfriend and implies that she's going to try to steal Scott from her. Allison aims her mini-crossbow at Erica and fires off a bolt, which Erica catches in midair as Allison predicted earlier. When Erica patronizingly asks Allison if she really thought that would work, Allison smirks and says that she did just as the Beta looks down at her hand and realizes the arrow was coated in Kanima venom, paralyzing her.
After the battle ended, Scott drags Erica and Isaac, who are paralyzed and unconscious, respectively, outside and tosses them at Derek's feet, causing him to become embarrassed by how Scott and his friends managed to get one over on them. Derek then reluctantly admits that he's figured out why Scott keeps refusing him-- he isn't an Omega after all, he's an Alpha of his own pack. However, Derek adds that Scott and his friends can't beat him, a powerful and experienced Alpha Werewolf, as he didn't know that Lydia had successfully called the police to the house, which forcing him to run with his Betas as sirens were heard heading their way.
Once the Hale Pack left, the Kanima appears out of nowhere, having crawled out of a second story window and climbed onto the roof. When Allison, Scott, and Stiles notice the Kanima, who hisses threateningly at them, the trio initially believe that Lydia has transformed into the Kanima just as Derek theorized. This assumption is then immediately proven to be false when Lydia bursts through the front door and demands that someone tell her what is going on, revealing that it is, in fact, Jackson who is the Kanima.
After Kanima-Jackson flees the seen, he catches up with a car parked on the side of the road and places its taloned hand onto the glass of the driver's door just before the mysterious driver places his own hand on the same spot from the inside of the glass.