Allison and Lydia are going to see Scott in hopes of finding out what the marks mean. Lydia isn’t convinced that it has a specific meaning, but Allison is positive.
Stiles and Scott are walking up to a house party that will be attended by kids from a different high school. Apparently Stiles was friends with the host when they were really young, and she promised to introduce them to her friends. The boys are vowing to move on from Allison and Lydia. Scott has a missed call from Allison, but he ignores it. Inside, the girl and Danielle are discussing sex. She wants to lose her virginity that night, even though her friend is telling her that it probably won’t be pleasant the first time. Stiles and Scott walk in, and Stiles starts to greet her, but before he can finish the girl has started kissing him. She takes Stiles downstairs, and Scott tries to approach Danielle but is ignored. His phone goes off again, and it’s Allison. The girl brings Stiles down to the wine cellar before starting to kiss him again. She tells him that she’s seventeen now, and what she wants for her birthday is to not be a seventeen-year-old virgin. She takes off her shoes and starts kissing Stiles again. She ask him if he wants to have sex her, and he agrees that yes, he would. Stiles doesn’t have a condom, but goes upstairs to get one from her brother’s bathroom. Allison and Lydia arrive at the party and meet Scott outside to show him the marks. Stiles digs through the bathroom until he finds condoms. They are size XXL. Stiles’s friend walks around the wine cellar waiting for Stiles to return with the condom when the shelves in the cellar begin to shake. Bottles of wine begin to fall off the shelves almost like they’re being pushed, before glass begins to shatter across the cellar as more and more bottles break. She moves against the wall to avoid the broken glass cutting up her feet and there is momentary calm before the window above her head is opened and she’s taken away. Stiles returns to the cellar and the girl, Heather, is gone. Stiles sees her missing shoe, but the glass and blood is suddenly gone and the window shuts silently.
Isaac paces in front of the window in Derek's loft talking to Derek about how he doesn’t like this idea. Isaac is especially uncomfortable about the fact that it has to be “him”, presumably Peter Hale. Isaac trusts Scott, who definitely does not trust Peter, but he also admits that he trusts Derek. Isaac says that he still doesn’t like him though, and Derek replies that “Nobody likes him.” The door opens and Peter enters. Peter tells them that he’s hoping that they’ll say whatever they think to his face, having overheard the previous conversation on his way in, and Derek replies with “We don’t like you, now shut up and help us.” Peter seems slightly amused by this. Peter discusses how easy it would be to kill Isaac accidentally, and catches Isaac off guard before sinking his claws into Isaac’s neck, hoping to recover Isaac's buried memories. Derek looks concerned as Isaac fights. Peter sees Boyd through Isaac’s memories and fighting, but everything is severely hazy. Someone is watching it all go down. Peter reveals that Isaac saw Erica and Boyd as well as Deucalion. Isaac asks what this all means, and Derek seems to think that he’s going to kill them, but Peter thinks it’s something else. Whatever Deucalion’s plans are, they would end with Erica and Boyd dead by the full moon, which is the next night.
Scott bring Lydia and Allison together with Derek at the school, but Derek doesn’t recognize their marks as any type of symbol. Derek is not impressed that this is Scott’s attempt to help, pointing out that in the spring Lydia resurrected his psychotic uncle and Allison tried to kill him and his pack. Stiles comments that there was no death, which is an “important distinction,” but Allison comments that her mother died. Derek fires back that it was her family’s honor code that killed Victoria, not Derek himself. Scott tries to advocate for the girls, and Derek’s response is that maybe Scott should actually tell Allison what the situation was when Derek bit her. Scott and Stiles walk into school, discussing why the alphas would want Erica and Boyd. Scott sees the twins and recognizes them as Werewolf, but doesn’t tell Stiles this.
In Business, Coach asks about the principles of the stock market and is really confused when Scott actually has the correct answer: risk and reward. Stiles goes to get a quarter for Finstock to use in an object lesson, and the condom from the night before that was still in his pocket falls out. Finstock makes a smart remark but takes the quarter. Stiles is about to play Finstock's game, make a shot for less work in the class, miss and you get extra, when his dad walks in. He needs to talk to Stiles about what happened to Heather. Apparently she’s now a missing person. Scott overhears the conversation just as Danny wins the quarter game.
Allison and Lydia are studying in the library and Lydia comments that she wants one of the twins, the straight one. Allison realizes that the mark could be a symbol not a logo looking at Lydia’s coffee cup as she exchanges numbers with one of the twins.
Scott and Stiles discuss reasons why Heather may have been kidnapped, and they’re lacking any motive. Stiles explains that it’s really important to him that they find her because their mom’s were best friends before Mrs. Stilinski died, and Scott states that they need Isaac to remember. They decide to consult Deaton. Scott, Derek, Stiles, and Isaac go to the Clinic, where they’ve decided to put Isaac in ice water until his heartbeat slows enough for Deaton to have access to his subconscious and find his memories. This happens to be slow enough for Isaac to be nearly dead. Derek tells Isaac that he doesn’t have to do it if it’s just too risky, and Scott agrees with the sentiment, but Isaac decides that it’s what he wants to do. Scott and Derek submerge him in the water, and he shifts trying to surface. Between Scott, Stiles, and Derek they manage to keep him down until he stops struggling before allowing him to float back up. Deaton tells the others that only he can talk or Isaac will get confused. He asks Isaac to remember, and Isaac begins to get upset saying he doesn’t want to and the lights flicker before Deaton gets him under control again. Isaac says he found Erica and Boyd in a stone building, maybe marble, and it’s dusty and empty. Isaac appears to feel like he is living in the memory and becomes very distressed when he realizes that the alphas can see him. Deaton manages to get him calm again before proceeding. A flashback is shown with Isaac stumbling around the building overhearing a discussion about the full moon and control. Isaac thinks he’s talking to Erica, but he can’t see to know for sure. Apparently the alphas are worried about what Erica and Boyd are going to do during the full moon. Derek comments that if they are locked in together without restraints they’ll probably end up tearing each other apart. Isaac suddenly becomes distressed again, repeating over and over that “they’re here” and Derek tries to ask him where is he. Isaac is beginning to panic more, and Deaton and Scott want to back off, put Derek forges on, knowing that he must have seen something particularly important to be reacting this badly. Isaac spews out several rapid sentences before yelling that it’s a bank vault before coming to. They others help him out of the water, and he reveals that it’s Beacon Hills First National and the other betas are locked in the vault. When all he gets are hesitant looks from the others and asks why, Stiles reveals that the last thing he said before the bank was that he was in a room where he saw Erica’s body. Derek insists that Erica can’t be dead, and Stiles asks who was the other person with Boyd in that case. Scott thinks it could be the girl who saved Isaac, but Isaac said that girl wasn’t a Werewolf, and the girl with Boyd was. Derek decides that he should get them out tonight in that case, and Stiles realizes that the bank was robbed and they could probably find out the details on how to get in pretty quickly.
The next morning the Sheriff walks in to find both Scott and Stiles asleep on piles of papers. They still haven’t heard anything about Heather, and after ten hours Stiles is frustrated by finding nothing useful about the bank. Scott insists on being optimistic about it, and right after his pep talk Stiles finds a new article with his dad arresting the robber, making him realize that his dad might know about the case.
Allison tells Lydia about her discovery that the symbol on their wrists in the logo of the abandoned bank. She tells Lydia she can’t bring her home, and there are bolt clippers in the trunk. Stiles and Scott discuss the plan, which is to meet Derek at his apartment at five.
Allison appears at the bank and enters, right as Stiles explains how the robbers managed to get in, with a map. Derek decides that he can punch through the wall, and uses Stiles’s outstretched hand for a demo, to Peter’s amusement and Scott’s annoyance. Peter doesn’t think it’s worth it to put together a rescue mission at all since both of the members of Derek's search and rescue team are unable to fight right now, but Derek refuses to do that. Derek asks Scott for his help, and Scott agrees that he’ll try, although he’s focused on who the other girl with Boyd is. Inside the vault, Boyd appears distressed. With him is an unfamiliar brunette. Allison walks into the bank in the darkness, looking around. A figure pins Allison to the wall. It’s Ms. Morell. She tells Allison that something big’s about to go down and she needs to get in the storage closet and lock it before it’s too late. The Alphas walk through the bank, and the female thinks she’s smelled something, but Allison spills ammonia in the closet, throwing her off. Looking around the closet with her phone’s flashlight, Allison finds Erica’s body.
Stiles is standing by the window in Derek’s loft, with Peter, rambling out of his own nerves. He attempts to discuss theory, but Peter isn’t very helpful until he suddenly has an insight about what the walls are made out of in the vault. Derek and Scott reach the vault, and Scott’s worried that there’s a bunch of risk that they can’t evaluate. Derek is sold on risking his life for theirs and gives Scott an opportunity to back out, but Scott follows him. Peter realizes that the material of the vault is important and has Stiles call Scott. Derek punchs out the wall as promised, and he and Scott enter the vault. Derek tries to approach Boyd as Stiles and Peter has explained that Derek’s already out-of-control betas haven’t felt the moon in three months, meaning they are going to be even more out of it than usual. Derek recognizes the brunette as "Cora", and seems oddly vulnerable upon recognizing her. Morell closes the mountain ash circle around the vault and the wolves shift, finally able to feel the moon. Ms. Morell is working with Deucalion.
Allison hears the fighting in the vault and runs out of the closet. It’s revealed that Cora is Derek’s younger sister who was presumed dead. Allison watches the fight, shocked, before breaking the mountain ash seal on the vault against Derek’s warning, and both Boyd and Cora run out. Derek is furious, explaining that now the two of them are going to go kill anything they can find. Allison refuses to feel guilty, telling him that she’s not the one turning teenagers into killers. He replies that “No, it’s just the rest of your family.” Derek mentions her mother, and Allison doesn’t understand. Derek tells Scott that he needs to tell her why exactly Derek felt forced to bite her. Allison, never realizing there was anything to tell, wants to know as well. At Lydia’s house, Lydia is asleep sitting in bed before suddenly starting to scream.