The doctors work with the Center for Disease Control and Prevention to find an antidote to the deadly viral outbreak at Angels Memorial, as Season 2 ends. Meanwhile, Leanne makes a life-changing decision; and Jesse welcomes a new batch of residents.
The ER is quarantined when a viral outbreak jeopardizes the lives of the doctors and patients at Angels Memorial. Meanwhile, Ariel, the young girl Leanne bonded with a year ago when she lost her father, returns to the hospital.
Mario risks his life to save two brothers at a construction site who are trapped on a crane 300 feet above the ground. Meanwhile, Leanne treats the wife of an orthopedist from Angels Hospital who is brought into the ER with mysterious injuries.
A code black at Angels Memorial takes a turn for the worse when a city-wide blackout leaves the hospital without power or back-up generators. Campbell and Leanne divert high-risk patients to a nearby care center, while Elliot and Heather become trapped in an elevator with a woman in labor; and Willis and Dr. Nolan try to help a 92-year-old woman who think it's 1942.
A 13-year-old girl arrives at Angels Memorial following a seizure caused by spina bifida and the doctors are shocked to learn she is Campbell's daughter. Meanwhile, a workaholic deaf patient and her translator are treated after their car rolls into the Los Angeles River.
Willis, Leanne and Jesse risk their lives to operate on an injured officer who has an explosive bullet lodged in her leg. Meanwhile, Mario, Noa and Guthrie treat a magician who comes to the hospital with breathing difficulties.
Willis and Campbell are sent to a Russian submarine in U.S. waters to treat a sailor who was badly wounded during an explosion. Meanwhile, violent convicts injured during a prison riot descend upon Angels Memorial.
Malaya must inform a college student who regains consciousness at Angels Memorial that she's the victim of a rape. Meanwhile, Willis and Campbell butt heads over a terminally ill woman who wants to end her life on her own terms; and a comedian is treated after he collapses on stage during a performance.
Willis defies Campbell's orders when he helps Mario and Heather perform a radical procedure on a maintenance man trapped under a boiler that exploded. Meanwhile, Angus' father tries to seize Power of Attorney over Mike, who is in a coma, even though Mike granted that right to his brother Angus.
Willis helps a young soccer player who is seriously injured in a bus crash make a life-altering decision. Meanwhile, a patient's hesitation to share that she's transgender makes it difficult for the doctors to diagnose her severe abdominal pain.
Season 2 begins with Col. Ethan Willis, from the Department of Defense, joining the team at Angels Memorial and taking Dr. Mike Leighton on a harrowing helicopter ride to save shark-bite victims in Malibu.