A member of Firehouse 51 clings to life and takes a sudden turn for the worse as medical staffers race to give treatment. Meanwhile, Dr. Natalie Manning prepares for her pregnancy and members of Chicago P.D. probe the case of a woman pulled from a fire.

The crossover starts on Chicago Fire S04E10 The Beating Heart (I). It continues on Chicago P.D. S03E10 Now I'm God (III).

Dr. Manning returns from maternity leave and treats a teen with a history of medical issues. Meanwhile, Dr. Choi cares for a former Navy shipmate; a Saudi prince arrives for a surgical procedure, but only from a hotshot surgeon; and a romance blooms between Sarah and a lab tech.

A husband makes a request following his wife's death; Dr. Charles tries to reason with a patient who wants to remove his own arm from his body; a child arrives at the hospital after swallowing magnets; Sarah has last-minute thoughts about her residency.

An erratic patient arrives, claiming that his heart is pounding inside his head as he takes drastic measures to stop it. Also, Dr. Manning helps a boy with a broken wrist, but she suspects deeper issues exist; Dr. Rhodes impresses with his skills; and Will receives news about a lawsuit.

Dr. Rhodes treats his mentor and Goodman faces personal issues in the Season 1 finale. Also: April deals with a disorderly patient; Dr. Manning's past catches up with her; and Sarah's graduation day arrives, but she remains conflicted about her future.

Dr. Rhodes begins a fellowship in cardiothoracic surgery, only to have problems with his new boss. Dr. Choi is the new chief resident. Dr. Charles approaches Sarah with an interesting opportunity that catches her off-guard. April faces her recently discovered illness.

Dr. Manning and Dr. Rhodes separately treat complicated cases involving sick babies, only to uncover that the two cases could actually be intertwined. Elsewhere, Dr. Halstead and Dr. Charles treat a homeless man with an upbeat disposition who sheds some valuable insight that prompts one of the doctors to learn a thing or two along the way. Dr. Choi has a Navy corpsman follow him for the day as they deal with the toll of gang violence in the city, while Dr. Reese tries to understand the delicate balance required in her new position.

A car accident that occurs at the Chicago Marathon leaves a victim in critical condition and tests the mettle of Dr. Halstead, April and her fourth-year medical resident brother Noah, who must perform a difficult procedure in the field. A mother brings in her 8-year-old girl experiencing a sudden hearing loss and Dr. Manning is left to tend to the case, which takes a complicated turn. Dr. Choi deals with Olga, a woman who is malnourished, prompting him to bring in Dr. Charles to help assess the case. Meanwhile, Goodwin takes measures to get her personal life in order.

Dr. Manning tends to a young cancer patient and struggles to inform the parents of her dire condition. Elsewhere, Dr. Reese gets contacted by Danny, the troubled young man, and turns to Dr. Charles and Det. Lindsay for help in resolving a difficult situation. Goodwin and Maggie do their best to clear the logjam in the hospital and Dr. Rhodes fine-tunes his instincts after both Dr. Latham and Dr. Halstead pick up on things he doesn't. Meanwhile, a teenage science geek and her friends impress Dr. Choi and Natalie with their bio-engineering lab.

Dr. Halstead and Dr. Manning deal with two warring brothers who are brought together when one finds himself in desperate need of a kidney. Dr. Reese's former patient Danny comes to a crossroad. Dr. Rhodes and Dr. Latham treat a 16-year-old girl with a genetic condition in need of surgery, but whose overprotective mother disagrees with that course of action. A convict with unusual intentions puts himself in the hospital and Dr. Charles meddles in his daughter Robyn's personal life, much to her dismay. Meanwhile, Dr. Halstead and April make a discovery that complicates her treatment for TB.

Dr. Rhodes and Dr. Charles' heart transplant patient gets into a car crash, revealing issues that could complicate her surgery. A past arrest causes trouble for Maggie when the arresting officer is brought to Chicago Med with a serious injury. Elsewhere, Dr. Manning and Dr. Halstead must defend their actions when explaining a patient's cause of death. Meanwhile, Dr. Halstead attempts to treat an uncooperative horse jockey.

A hacker demanding a ransom payment shuts down the hospital computer system, forcing the doctors to diagnose patients without the aid of technology. A stressed Goodwin struggles to keep the hospital afloat when an unexpected visit from her ex-husband Bert and his new girlfriend make matters worse. Making use of his former navy experience, Dr. Choi and Noah have to locate a bullet inside a patient with a gunshot wound. Meanwhile, Sarah feels Dr. Charles may be trying to push her out and Robin's unusual behavior worries Dr. Rhodes.

A new hotshot doctor arrives at Chicago Med causing competition for Dr. Rhodes, Dr. Halstead and Dr. Manning work together to diagnose a Syrian refugee without a medical record. When Robin is admitted back into the hospital for more complications, Dr. Reese takes a different approach to her treatment. Distracted from work because of Robin's worsening case, Dr. Charles is challenged by Goodwin and Noah hosts a graduation party that brings everyone together.

When a sick boy is brought in, Ethan is faced with a tough decision that quickly escalates into a complicated and dangerous situation. Connor and Maggie are put to the test, while Natalie and Ethan aren't sure what to make of Elsa, the new third year medical student.