Casey finds himself in dangerous territory at the center of an undercover operation to bring down a trafficking ring out of Nesbitt's strip club.

Internal Affairs investigates Firehouse 51 following a complicated call to a fire.

Boden's job is on the line as the crew of Firehouse 51 continues to search for evidence that could clear him of any wrongdoing.

A member of Firehouse 51 requires the support of the whole team to recover from a near disaster.

Severide and Patterson's differences reach a boiling point, while a visitor from Cruz's past asks for a favor.

Boden finds himself in the middle of a fiasco concerning a female neighbor he believes is setting him up.

Patterson confronts Boden about his ability to lead the firehouse.

Clues to Serena's possible whereabouts are uncovered, while Captain Patterson has news for everyone.

While a member of Firehouse 51 falls victim to a near-fatal stabbing, the other firefighters rescue a woman who attempted to commit suicide.

The city of Chicago braces for a tornado.

A tormented Herrmann is forced to make a difficult decision and Casey does his best to keep a shelter open that's housing recent tornado victims.

Mysterious threats begin to emerge throughout Chicago and the situation gets worse when news of a mass shooting brings Firehouse 51 into a dangerous scene.

Severide confronts Chili about her behavior, while Jimmy faces Antonio in the first annual "Battle of the Badges".

Severide and Cruz take matters into their own hands when Firehouse 51 responds to a suspicious call from a neighboring firehouse.

A citywide propaganda attack against Casey's run for alderman makes him reconsider his candidacy.

Severide and Cruz discover the remains of a case that may be connected to a case Severide worked on years ago.

Sylvie is threatened when she witnesses a murder.

Casey tries to use his political influence when a high school student is almost killed by a street gang.

Severide watches over Detective Holloway's son while she testifies in court as part of an undercover operation, but he's left to deal with the situation due to an unexpected turn of events.

Dawson risks her life to save a young boy from an apartment fire.

Firehouse 51 heads to the scene of a building collapse where dozens of lives are on the line.

Dawson hopes to foster Louie while a dangerous structure fire puts a member of Firehouse 51 in a dangerous situation.