John Babbo

An arsonist targets Lt. Severide. Meanwhile, Station 51 is under scrutiny by Gail McLeod, a financial consultant for the city who is deciding if any firehouses should be closed. Mouch receives a visit from his Japanese girlfriend, Lt. Casey "celebrates the one-year anniversary of Andy's death, and Shay suspects Severide's paternity.

Boden's (Eamonn Walker) future with Firehouse 51 is on the line and McLeod (guest star Michelle Forbes) keeps on him about his past wrongdoings, pushing him toward an early retirement. With Boden considering his options, McLeod calls on an unlikely candidate to potentially replace him as Chief. Meanwhile, it becomes clearer who the mole inside the firehouse is and everyone takes measures to make their disdain for him known. Meanwhile, Shay's (Lauren German) social life is in full bloom and Arthur (guest star Mike Starr) continues to take money from the struggling Molly's.

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 (Eamonn Walker) is tasked with making life-changing decisions regarding his future. Severide (Taylor Kinney) learns more about Katie (guest star Brittany Curran), the young woman he saw with his father (guest star Treat Williams). Meanwhile, Shay (Lauren German) and Dawson (Monica Raymund) are still at odds and Casey's (Jesse Spencer) responsibility of being a surrogate father takes a sharp turn.

Lt. Severide (Taylor Kinney) has his strong ideas on who the serial arsonist is, but first needs to find proof to back up his suspicions. Meanwhile, Lt. Casey (Jesse Spencer) adjusts to his new lifestyle in the wake of recent events and Molly's bar is failing, much to the dismay of owners Hermann (David Eigenberg), Dawson (Monica Raymund) and Otis (Yuri Sardarov). Elsewhere, Dawson goes out on a date with Jay Halstead (guest star Jesse Lee Soffer), one of the bar regulars, and Mouch's (Christian Stolte) campaign trail starts to get on track with the aid of Isabella (guest star Mena Suvari), an old friend of Dawson's.

In the wake of Heather's (guest star Chaon Cross) incident, Lt. Casey (Jesse Spencer) is having a hard time handling his newfound responsibilities. Lt. Severide (Taylor Kinney) is on edge dreading the next attack from the firehouse’s arsonist. Meanwhile, with Renee (guest star Sarah Shahi) back from her mother's, Severide knows he has to confront her about his paternity suspicions. Elsewhere, Mouch (Christian Stolte) begins his campaign for union president and Shay (Lauren German) and Dawson (Monica Raymund) feel that they need to get out more.