When Stella's music box disappears at school, the gang immediately suspects a student urban legend known as the Rat Beast.
Lincoln, Clyde, Stella, Rusty, Liam and Zach are on the bus heading to school. On the ride, each one of the friends are presenting something to show and tell for the class, as today is Friday. Stella presents her item: a music box with a small dancing elephant.
In Mrs. Johnson's class, Stella is prepared to showcase her music box, but as she digs through her backpack, she discovers that it's missing. The boys quickly defend Stella by telling Mrs. Johnson that they indeed see Stella bring her music box. During lunch, Stella can't stop thinking about her music box. In secrecy, Zach makes the assumption that it was stolen by the Rat Beast. This assumption horrifies the boys and explain to Stella that it's a student urban legend about three lab rats mutated into one after an experiment gone wrong in the science lab. They describe the Rat Beast as huge, having glowing green eyes, many tails as well as creating disturbances around the school such as things going missing, making noises in the hallway and leaving the smell of corn chips in the vents. Now believing that the Rat Beast is responsible for her music box going missing, Stella decides to get more information, to which the boys join in and help, despite being hesitant at first.
Stella goes around the school asking other students if they know anything about the Rat Beast, and learns that it stole other kids' belongings, such as Girl Jordan's hoop earrings and Mollie's lucky Canadian penny. As for the boys, they go around the school hoping to find some clues to the Rat Beast's whereabouts, but all their leads end up hitting dead ends. When the gang meet back up and announce their lack of clues, they suddenly hear the music box's music in the walls and proceed to chase after the sound. After a near run-in with Principal Huggins, the gang track the noise to the school's basement and proceed to head down.
As the gang searches the basement, they find what they believe is the Rat Beast, but it turns out to be Norm, and inform him that a creature known as the Rat Beast is lurking in the basement. Not wanting a giant rat to roam around the school, he decides to join them on the hunt. As Norm heads one direction and the gang takes the other, the gang suddenly hears Norm fighting something in a supply closet and quickly rush up to him. Norm exits the closet, beaten up, stating that he managed to take the Rat Beast down, but couldn't find Stella's music box. Just as they are about leave, the Rat Beast suddenly arrives and the gang is prepared to attack it. However, Norm stops them, revealing that the Rat Beast is just a normal sized rat with glowing green eyes and two tails named Cinnamon, as well as revealing that he was play acting in the closet in an attempt to drive the gang away. He proceeds to tell the gang the story of Cinnamon.
Ten years ago, Cinnamon was just a normal rat who liked to keep Norm company in the science lab while he worked the late shift. His favorite snack to eat was corn chips, which explains why the vents smell like that. However, one day, Norm forgot the close the lab's supply closet due to having too much fun, and when Cinnamon attempted to reach a bag of corn chips on the top shelf, he ended up mutated from the chemicals stored. Out of fear of scientists taking him away to be studied, Norm kept him hidden in the basement the whole time. After explaining the story, the gang reveals that Cinnamon has been stealing the student's possessions, and Norm orders him to return all of the items. Seeing how much Norm goes through to protect Cinnamon, the gang decides to keep it a secret between all of them.
Sometime later, the gang gives back all of the stolen possessions and tell them that they simply found the items scattered around the school and that Rat Beast was just a hoax. As the gang prepares to swap lunches, Rusty attempts to trade his mustard sandwich for Stella's corn chips, but she refuses, intending to share them with Cinnamon, who is currently hiding in the vents.