Donna believes students started the fire at the school. Boden wants Severide and Casey to investigate. 

They get called to a bad crash with a fellow firefighter stuck under a truck. They pull him out and Herrmann nearly comes to blows with a member of the other department. Mouch makes a $1000 bet that their house will win the Muster challenge. 

Severide and Casey go to the school, but can't confirm how the fire started. Boden tells them to keep digging, but they wonder if Donna's hunch could be wrong. 

Casey and Severide go back to the school to question the chemistry students. One kid, Mateo, is clearly avoiding talking to them. 

Brett and Dawson are called to treat a kid from the school fire who has been beaten. He doesn't tell them who attacked him. 

Severide gets Mateo to admit to assaulting the kid and starting the fire, but Severide doesn't think he actually did it. He claimed he lit a match, but it was a chemical fire. 

It turns out the kid who was injured and beaten was actually the one who started the fire. He was trying to get Mateo kicked out of school, but did not expect it to get so big. 

Cruz finally tells Mouch he can't do everything he says anymore. 

51 wins the Muster.