Firehouse 51 responds to a fire at a warehouse that has been converted to a living space. The warehouse is old and poorly maintained with a large number of people inside. While attempting to rescue a young woman a railing collapses and traps Mouch underneath.
That leads Boden to order an evacuation of the building, just as Alvin Olinsky arrives and tells the chief that his daughter Lexi was inside. Boden and Severide find her unconscious and badly burned in the back of the warehouse. Overall, almost 40 people die in the fire or from injuries resulting from it.
The subsequent investigation originally targets the building's landlord, who commits suicide in the Firehouse 51 bathroom after Casey and the OFI investigator remind him of his role in the situation. But later on it's uncovered that it was arson, leaving the team to wonder who could have started such a terrible tragedy.
Meanwhile, Brett and Dawson try to reunite a young man with the girl he'd been dancing with when the fire broke out, and Anna comes to Chicago in order to support Severide.