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).

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.

Goodwin's troubles persist. Charles clashes with his former resident over a depressed patient. Lenox and Ripley treat a car accident victim who recently reunited with his college sweetheart.

Asher treats a woman whose undiagnosed condition threatens the lives of both her and her unborn child. Goodwin receives a chilling message while battling the hospital higher-ups.

Gaffney gets a surprise visit from a National Accreditation Board of Hospitals representative. Hannah and Naomi search for the cause of a young girl's infection. Lenox and Frost work together to save the victim of an accidental shooting.

An army of surgeons prepares to separate conjoined twins. Asher reconnects with someone from her past. Archer treats a tyrannical boss with a heart problem.

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.

When a commuter ship capsizes, the ED finds itself under water as the staff struggles to treat the influx of new patients and deal with bold new employees taking charge.

Lenox continues to rankle Archer as the two tackle their new roles. Ripley faces possible suspension over what really happened to Pawel. Goodwin is forced to make layoffs.

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.

Chicago P.D. steps up Goodwin's security after she's threatened online by a former employee. Archer and Frost treat victims from a disciplinary camp injured during a rockslide. Maggie gets roped into delivering a high schooler's baby.

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.

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.

Ripley struggles to rescue a woman and her daughter trapped in an abandoned well. An old friend from Frost's TV days shadows Maggie. Asher discovers an extremely rare condition in her patient's ultrasound.