In the wake of Heather's incident, Lt. Casey is having a hard time handling his newfound responsibilities. Lt. Severide is on edge dreading the next attack from the firehouse's arsonist. Meanwhile, with Renee back from her mother's, Severide knows he has to confront her about his paternity suspicions. Elsewhere, Mouch begins his campaign for union president and Shay and Dawson feel that they need to get out more.

When a tanker collides into a commuter train that subsequently crashes into a warehouse, the firehouse is called to the horrific scene that tests everyone to the limits as life and death hangs in the balance. With the ultimatum in place for him or the firehouse, Boden is tasked with making life-changing decisions regarding his future. Severide learns more about Katie, the young woman he saw with his father. Meanwhile, Shay and Dawson are still at odds and Casey’s responsibility of being a surrogate father takes a sharp turn.

Severide returns to Firehouse 51, but he is clearly not his usual self. Dawson counts down the weeks until she gets started as a candidate at the Austin Firehouse. Mills learns more about his father’s family. Herrmann asks the rest of the Molly investors to come up with some new ideas to turn the business around.

Truck and squad are called to the scene of a car crash that has Mills desperately working to save the life of a driver seriously injured in the wreck. Meanwhile, Casey has some doubts about Jack Nesbitt, whose strip club he is remodeling on the side. Elsewhere, Hermann, Otis, Cruz and Chaplain Orlovsky help get their team ready for the peewee hockey championship, but Herrmann might prove more of an obstacle than the other team.