Boden offers to host a wedding at the firehouse after a fire ruins the ceremony, but Chief Riddle is against it and his political aspirations cause friction among the firefighters.
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 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.