A power outage at Med puts multiple patients at risk and tests the skills of the staff and their enthusiastic med students; Natalie returns following her traumatic brain injury; Ethan and April wonder if a family is in their future.

Will and Stevie get stuck in the middle of a marital face-off. Dr. Blake's entire surgical team mysteriously collapses during a liver transplant. Charles encourages Ethan to reconcile with his father.

Halstead and Cuevas struggle to diagnose a patient with a rare disorder. CFD and Med staff work to free a paranoid father trapped in the MRI suite. Tanaka-Reed gets an ego check.

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).