An infection threatens Marcel's risky liver transplant. Maggie and Charles help a struggling burn unit nurse. Goodwin is forced to make a tough decision about Bert's future.

A familiar face returns to the ED. Ripley and Charles treat a woman with a rare disorder. Frost suspects that his patient's father is lying about his daughter's condition.

Halloween chaos takes over the ED. Asher helps a pregnant woman with an autistic son. Charles clashes with Nurse Jackie on a psych case.

Ripley and Frost uncover shocking information that conflicts with their patient's DNR. Asher facilitates an unexpected reunion. The tension between Archer and Lenox comes to a head.

The team fights to save the life of one of their own. Frost and Abrams butt heads with their patient. Asher helps a religious young woman with an ectopic pregnancy.

Following the subway tunnel collapse, Ruzek and Kidd treat injured passengers and track down the missing offender. Archer and Frost perform a life-saving amputation.

The crossover starts on Chicago Fire S13E11 In the Trenches (I). It continues on Chicago P.D. S12E11 In the Trenches (III).

Goodwin struggles with the psychological aftermath of her attack. Ripley treats a patient with helicopter parents. An ED staffer makes a mistake that may cost them their job.

Lenox's mentorship is put to the test. Asher helps a religious young woman with an ectopic pregnancy. Ripley's self-destructive behavior continues to escalate.