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.
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