Out they scampered from doors, windows, and gutters, rats of every size, all after the piper.
The audience at a rave cheer for the DJ, Retchid Kat, to come out. Backstage, the rave organizer, Sammy finally sends out Roddy Geiger, who dons a cat mask and goes out to perform. One of the partygoers texts her friend, Sarah Jessup, who is practicing at the Van Hamelin institute with the other three boys in her string quartets. Their teacher, Paul Lawson, focuses on one of the boys, Carter Brimley, and tells him to shape up so they’re ready for the performance on Saturday.
After the practice, Lawson goes out to his car and gets in. As he starts up the car, he sees hundreds of rats in the back seat. They swarm over into the front and crawl over him, biting. The teacher, in agony, is unable to get the car door open.
The next day, assistant principal Grace Kaplan arrives at the institute early and sees Lawson’s car. She goes over to see who is inside and screams in horror. Meanwhile, Nick is at home with Juliette and they discover that their refrigerator has broken down. Nick gets a call summoning him to the institute. He and Hank arrive there and Wu informs them that the dead man is Lawson, the music teacher. They’ve kept the car locked until Animal Control arrives. When they’re ready, the officers open the car door and the rats swarm out. The detectives check the nearby bushes and find several rats cage with the logo of Geiger Pest Control.
Nick and Hank go to the home of Sarah Jessup and meet with her and her friends, Carter, Trey, and Marvin. The officers ask if they saw anything unusual after the practice ended, but none of them can report anything useful. When Hank mentions the cages belonging to Geiger, Carter says that Roddy Geiger goes to their school but was suspended. When Nick tries to find out why, Sarah’s mother tells them that they should talk to the school administrators because it’s not appropriate for the kids to comment.
The detectives visit Ephram and his son Roddy at their trailer on the river. They hear music and investigate, and find Roddy playing the violin for an audience of caged rats. Ephram comes over and angrily demands to know why they’re there. He insists they had nothing to do with Lawson’s death, and Roddy is surprised to hear that the teacher is dead. Ephram continues yelling at them and Hank grabs Roddy when he tries to run. As they argue, Ephram briefly transforms into a rat-like creature. While Hank restrains him, Nick runs after Roddy and easily brings him down. Roddy also briefly transforms into a rat form, and knows that Nick is a Grimm. He figures that Nick wants to kill him and charges, but Nick knocks him down and says that he doesn’t want to kill him.
At the station, Hank interrogates Ephram, who says that he was in the mountains releasing rats but doesn’t have an alibi. He also can’t provide an alibi for Roddy, and says that the cages were stolen but he didn’t bother reporting them. Ephram tells Hank that the students never let Roddy forget that his father is an exterminator, even though he was a prodigy. Hank suggests that Roddy was angry at Lawson and snapped. Meanwhile, Nick talks to Roddy in another room. Nick tries to sympathize but Roddy refuses to listen to him because he’s a Grimm. Roddy says that he attacked Carter only because he had to fight back. When Nick asks where he was, Roddy admits that he was at a rave and that the organizer, Sammy, can provide an alibi for him.
Later, Wu talks to Sammy and confirms Roddy’s alibi. He tells them that Roddy is Retchid Kat and is a famous DJ. Hank points out that Ephram still doesn’t have an alibi, but Nick doesn’t think he would have left the cages behind. They let Roddy go but keep Ephram, and then meet with Dr. Harper. She tells them that Lawson died from a heart attack and the officers figure that it was a prank gone bad. Harper has also done an autopsy of one of the rats and confirmed that it had eaten colored cloths that didn’t match what Lawson was wearing. Nick figures that they were chewing on something in the car that was used to transport them. Renard calls the officers in and tells them to close the case, and warns that the rich parents at the private school want answers.
Roddy goes to Sarah’s home and waits until she comes home. He approaches her and tells her what happened to Ephram. Roddy figures that someone is setting him up and Sarah points out that he was suspended. When Roddy complains that she didn’t say anything, Sarah says that she stood by her friends and only used to care about Roddy. He starts to kiss her, but Sarah says that she can’t keep sneaking out. She wonders who put the rats in Lawson’s car and Roddy figures that Carter did it to get rid of her. Mrs. Jessup comes out and orders Sarah into the house and tells Roddy to stay away from her family. As Roddy leaves, Carter pulls up and taunts Roddy. Roddy warns him that he’ll pay and walks away.
While Renard gives the press an update, Nick and Hank get the lab report on the fibers. They’re from expensive cars, not the one that the Geigers drive. Mrs. Jessup comes in and demands to know why Roddy isn’t arrest. She tells them that Roddy harassed her daughter and tells them to get some evidence to put Roddy away. Mrs. Jessup tells them that Roddy used to see Sarah and insists on filing a report against him. Afterward, Hank heads out for drinks with Wu. He realizes that Nick has a life and figures that he is going to spend time with Juliette.
Nick goes to see Eddie instead and they discuss the Geigers, who are reinigen, rat-like creatures. They are non-violent unless provoked, and Nick asks Eddie to talk to Roddy since Eddie is also interested in music. Eddie warns that reinigen and blutbaden aren’t drinking buddies and that he’s not a do-gooder. However, Nick appeals to him by saying that they’re both outsiders and Roddy needs some help as well.
Nick goes home late but the refrigerator repairman, Bud, is still working on the appliance. He cuts his hand and Juliette goes to get a bandage, and Bud turns around and recognizes Nick as a Grimm. Nick sees him as a reinigen and stares in surprises as Bud runs out in terror. Juliette comes back and wonders why the man was so scared, and Nick doesn’t have a decent explanation.
Hank and Wu are drinking and discussing Nick, and then Hank leaves early. He runs into Adalind Schade and they chat, and then she asks him to have dinner with him since her business meeting fell through. Out in the street, Renard watches the two of them go to the table.
Eddie comes to see Roddy and hears him practicing in his trailer. He comes in and introduces himself, and compliments Roddy in his music. Eddie reveals that he’s a blutbaden and explains that Nick isn’t a typical Grimm. He berates Roddy for throwing away his true talent, a talent that most people never have or even recognize. Roddy doesn’t have anything to say and Eddie gives him his card and tells him to call if he needs anything. Once Eddie leaves, the station calls Roddy to tell him that his father was injured when he refused to enter his cell, and is being held for 48 hours at the infirmary.
When Nick and Juliette return home, Juliette still tries to work out why Bud was afraid of Nick, and suggests the repairman is someone that Nick has arrested. Eddie calls Nick and tells him that he gave him something to think of.
At home, an angry Roddy smashes apart his room.
The next day, Hank and Nick tell Renard that they checked German cars belonging to students at the Institute, and that Carter owns one such car. Renard tells them to get a warrant.
Roddy goes to his rat collection and releases the animals, kicking over the cages. He grabs his violin and Retchid Kat mask and leaves, unaware that he knocked over a propane tank and started a fire.
At the institute, Carter meets with Trey and Marvin. Marvin worries that Roddy knows they pulled the prank, and Sarah asked him if they did it. Carter tells him to man up and then they all three get an invite to a rave with Retchid Kat.
Nick and Hank go to Carter’s home and talk to his father, Lawrence. They present the warrant and search Carter’s car, and find a blanket with chew holes. There are rat droppings in the car, and Lawrence refuses to talk to them until the officers threaten to get an arrest warrant. He tells them that Carter went with his friends to a movie, and the officers get a call about the fire at the Geiger property. They go there and realize that Roddy has gone berserk and will go after Sarah.
Roddy heads to a warehouse on the docks, with a horde of rats following him.
Nick and Hank go to the Jessup home. Mrs. Jessup thinks that her daughter went to the movies. However, when she calls Sarah, they realize that the phone is at home. Nick checks her call log and finds a call from Carter telling Sarah about the rave.
Sarah is with Carter and the others, driving to the warehouse. At the warehouse, Roddy has filled the basement with lit calendars and his playing his violin to summon increasing number of rats.
The students arrive at the warehouse and are surprised that no one else is parked outside. As they go inside, Sarah asks Carter if they framed Roddy for the fight. Carter sticks by his story and asks if she wants to be with them or Roddy. She goes in with them and they follow the music playing in the basement. Nick and Hank pull up a few minutes later and go inside after them.
When the students get to the basement, they find Roddy waiting for them wearing his DJ mask. He accuses them of killing Lawson and Sarah realizes that Carter and the others did it. They realize that Roddy is Retchid Kat and he summons the rats to attack them. Nick and Hank arrive as the boys run off and Sarah trips. Hank scares off the rats with gunfire while Nick pulls the rats off of her and watches as Roddy walks away.
Outside, the three boys insist that they just wanted to scare Lawson and that his death was an accident. Sarah says that Roddy was better than all of them and asks Hank to tell Roddy that she’s sorry. Meanwhile, Nick tracks down Roddy in the tunnels and tells him that Sarah knew nothing about the fight. He asks Roddy if it was worth it, and Roddy says that they wouldn’t have admitted anything if he hadn’t summoned the rats.
Back at the station, Nick brings in a pizza for Hank, who wonders what they can charge Roddy with. Nick says that no one will believe that someone summoned rats with violins and suggests they drop the entire thing. Hank wonders how Roddy summoned the rats but Nick dismisses it as a coincidence. Nick offers Hank the pizza box and the officer jumps back when he discovers that Nick has left him a rubber rat.
The next day, Bud comes back from his tools. Juliette is there alone and says that Nick doesn’t remember arresting him. Bud has no idea what she’s talking about, says that he’s a family man who hasn’t done anything bad, and tells her to make sure that Nick knows that.