The FBI takes over Fire House 51 while in pursuit of a criminal who killed an agent in St. Louis. The man's brother lives in the apartment building across the street, and they are using 51 as an observation base. This puts 51 on a "restricted duty" status -- they won't be going on any calls and their doors will be closed to the community. The team rankles at this, and things escalate between Cruz and FBI Agent Stanton when a young boy Cruz has offered to tutor is run off. The kid later throws a brick through the window in anger, and Stanton goes after him with his gun drawn; Cruz has to put himself between the boy and Stanton to protect him. But the disruption caused by the brick causes the FBI to miss their target's arrival. Stanton is all fired up to storm the building, but Boden convinces the lead agent, Riordan, to agree to Casey's plan. Casey calls a man they know who lives in the building and has him pull the fire alarm, prompting an evacuation that will keep the residents safe. He and Kelly then lead the FBI agents into the building, undercover as firefighters. Things don't go quite as planned though, and the man the FBI is hunting takes Casey and Severide hostage with his brother's help. The officers manage to body slam their captors and the FBI takes the two men out. 

Kelly is still uncomfortable with his parent's rekindled relationship. When his father accuses him of not respecting him or his mother, Kelly accuses Benny of using Jennifer because he's old and afraid of being alone. Benny ends up dumping Jennifer. When Kelly tries to take responsibility for the breakup, Jennifer acknowledges the role he played. She also warns Stella that Kelly is just like his father to watch out for the moods and the women. 

Despite being physically attracted to new-comer Cordova, Sylvie is impressed with the care Joe shows for Taye, a kid from the neighborhood. Her feelings for him seem to be rekindling, and she buys him a beer at the end of the episode, calling him a good guy. 

Gabby focuses on strengthening her relationship with Casey, but is shocked when Cordova tells her that their relationship wasn't "nothing" to him like it was to her.