Fred arrives home screaming and shares his flashback with the audience. The day before Fred went to music class to play Fur Elise and his favorite music teacher Mrs. Felson, who has trouble hearing, told him that he plays beautifully. The next day he comes in to play, expecting Mrs. Felson, but finds out that she has been replaced by Mr. Devlin. He then sees Mr. Devlin cleaning up and throwing away a hearing aide, which makes Fred wary. Later he is walking home with his best friend, Bertha, and sees a girl named Talia following them. Believing that he is being stalked, he runs home and arrives screaming. Later that day Fred notices Mr. Devlin is his new neighbor. Fred is immediately suspicious of Mr. Devlin, and begins to think that he may be a vampire.
The next day at school Devlin supports everyone to join a community called Join the World of Music. Fred thinks that Devlin is trying to turn everyone into a vampire by having them take his lessons, and to his horror everyone takes the lessons. That night Fred spies on him and sees him burying something, which he suspects is a body. After Fred finishes talking to Talia, she vanishes, causing him to think she is a ghost; he imagines her ghost in his house and falls down the stairs reaching for her. The door is knocked on and he opens it to see Kevin's mom and she invites them to a party; then it is revealed that Talia is Kevin's sister. The next day, the doorbell rings and Fred opens it to see Mr. Devlin, who is taking his mom on a date; this makes him become immediately uneasy.
Fred enlists the help of Bertha to lend a helping hand in getting enough proof that Mr. Devlin is a vampire. Fred and Bertha spy on Mr. Devlin and Fred's mom's date, with Bertha posing as a waitress . Mr. Devlin orders a bloody steak making Bertha more suspicous. He then spies on Mr. Devlin sees his old music teacher's jacket; he then becomes even more suspicious of Devlin. Later that night, he falls from his window because he notices Mr. Devlin see him as he is trying to get away. Then, his "dad" pulls him up and brings him to a wrestling arena, where they form a tag team. Mr. Devlin and Kevin also form a tag team. His dad beats up Mr. Devlin, Fred finally pins Kevin and him and his dad win. At this point, Fred wakes up, suggesting it was all just a dream. Then Fred goes around putting up crucifixes to fend off Devlin, as vampires hate all things holy. He also sees Derf, who tells him to face his fears and defeat Devlin. Fred then buys garlic sauce from a Chinese restaurant. At the piano recital that night, Fred squirts the garlic sauce on everyone, including Talia. Afterward, to Fred's horror, Mr. Devlin invites him into his house for dinner so that they can "bury the hatchet". Fred sets up a live video streaming from his phone so he can prove that Devlin is a vampire. Inside Devlin's house, Fred discovers a secret butchers' room behind a wall filled with meat and bones. As Fred investigates, Devlin creeps in with a long knife and a tall head-dress, scaring Fred; Fred then drops his phone into a pot of boiling liquid. The video freezes on Devlin, and the frozen image of Devlin in his headdress causes everyone watching the video to think that he is a vampire. But after Devlin explains every weird hobby he has, Fred starts to relate to him, and he comes to like Devlin.
At school he realizes that everyone has seen his video, which makes everyone believe that Devlin is a vampire. He runs to Devlin's house to apologize but Devlin is too depressed to answer. When Fred sees Mr. Devlin get fired, and put up a sign for his house, saying that he is moving, he feels guilty and tries to fix it all by making them think he is the vampire. Bertha and Talia both help, as Talia reveals how much she dislikes her brother. Kevin and his friends go up to Devlin's house because they think he has Talia. Devlin tells them that vampires are not real, but Bertha comes out and says, "Vampires are real, but it's not Devlin. It's him!" Then, Fred walks out of the garage carrying Talia, threatening to turn her into a vampire too. Bertha makes Kevin stab Fred, and he gets sprayed with fake blood. Devlin sees Fred and accepts his apology, revealing that he quit his job and sold his house of his own choice, but still sees Fred as his one true friend. In the end, as Fred's mom and Devlin go out for dinner and go out the door, Fred looks at the mirror in his house and sees that Mr. Devlin has no reflection, revealing that he was a vampire after all, and Fred screams in terror as the film ends.
