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.
Casey gets help from Susan Weller for an urgent personal matter; Severide is given an opportunity to travel with Travis Brenner; Stella saves a street kid from a tunnel fire.
Casey is forced to take extreme measures when a forklift accidentally pierces a tank of acid, causing it to spill and putting a father and daughter in danger.
Dawson hits a pedestrian while responding to a call. Casey and Severide continue to investigate an arson case while Chief Boden is in New York to attend an important function.
Casey and Dawson apply for permanent adoption of Louie. Jeff Clarke gives Severide an interesting proposition. Otis convinces Herrmann to throw a bash celebrating the 100th anniversary of Molly's, but there are complications.