Four new first-year residents are immediately put to work by the ER's tough-minded residency director Dr. Leanne Rorish.

Christa saves a woman's life and then pleads with Neal to perform a surgery that will save the patient's reproductive future. Meanwhile, Mario gets into a fight with a patient after he delivers a diagnosis that could end the man's hockey career.

The doctors work 36 consecutive hours in code black and must deal with a distraught mother of two sons who were in a devastating car accident. Meanwhile, Mario bullies Angus into performing an unnecessary operation and lets Angus take the blame when Leanne finds out.

A new surgical attending Cole Guthrie joins Angels Memorial with the help of his estranged father and clashes with Leanne over their contrasting approaches to medicine. Christa has a crisis of confidence in the middle of a procedure on her first infant since the loss of her son.

Neal clashes with his neurosurgeon father over how to treat his gravely ill mother, whose mysterious symptoms force Angels Memorial to quarantine Trauma One.

Leanne must deal with the difficult decision of whether to save Tom Moreno (Michael Trucco), the police officer who helped her during her family's accident, or his shooter, who has more serious injuries. Also, when Malaya's pregnant ex-girlfriend, Carla Niven (Shiri Appleby), arrives at the hospital with pain in her arm, Malaya is shocked by her MRI results.

Angus utilizes his background in psychology when a woman cannot remember how she injured herself and others; Christa bonds with the mother of a patient with cystic fibrosis.

Christa must handle all emergencies at Angels Memorial that happen outside the emergency room; Malaya arranges a bone marrow drive for Carla; Leanne turns down a date.

When the lead actor of the musical “The Lion King” tour arrives at Angels Memorial with a throat infection, Leanne gives him a diagnosis that puts both his career and life in jeopardy. Also, the doctors and the cast of “The Lion King” help a 17-year-old boy deal with the difficult decision of taking his father off life support,

Leanne becomes rattled at the thought of losing Jesse, as he fights for his life after suffering a massive heart attack. Meanwhile, Mario asks the residents to cover for him so he can search the city for a patient’s son, which goes against Neal’s direct order not to leave the hospital

Heavy fog at the scene of a massive multi-vehicle accident puts the doctors in danger; Jesse's confidence has taken a hit; Carla's condition turns critical.

As the doctors treat patients from the chaotic multi-vehicle accident, a love triangle begins to build between Mario, Angus and Heather. Also, when Malaya learns that Carla is at the hospital, she goes against Carla's wishes and stays by her side during the delivery of her baby. Shiri Appleby guest stars.

Angus' older brother, an "all-star" ER doctor, tries to secure a position at Angels Memorial. Meanwhile, Christa's romantic feelings towards Neal begin to grow; and Malaya deals with a patient who is obsessed with her.

Leanne visits a prison to finally confront the drunk driver who killed her family. Meanwhile, Malaya begins to feel uncomfortable around her patient.

After a violent incident inside the hospital results with some grave injuries, the doctors are questioned. Christa and Neal admit their relationship to human resources.

Christa is feeling insecure after Neal's former girlfriend makes a return to the hospital after a year of volunteering in Haiti.

Christa has to try and ignore her personal feelings as she and Grace attempt to help a patient who says he was abused at a camp for troubled teenagers.

Mario confronts Angus about his Adderall abuse and Christa is surprised to learn how romantically involved Neal and Grace were before she left for Haiti. Also, Dr. Mark Taylor returns to help Leanne find the money in their budget to keep Jesse and the nurses from walking out.

The new residents are introduced to the ER. A new doctor from the army who knows many advanced procedures is flown to a shark attack on the beach with Mike Leighton and Leighton falls 20 feet from the helicopter. Dr. Guthrie Jr. is promoted to the director of the ER in addition to the OR.

Willis must help a promising soccer player make a life-altering decision after he's seriously injured, and another patient's hesitation to share that she's transgender makes it difficult for the doctors to diagnose her severe abdominal pain.

Willis defies Campbell’s orders when he helps Mario and Heather perform a radical procedure on a maintenance worker stuck under a boiler that exploded. Also, Angus’ father, Dr. Desmond Leighton, tries to seize Power of Attorney over Mike, who is still in a coma, even though Mike granted that right to his brother, Angus.

On Halloween night, Willis, Noa and Elliott rush to the scene of a fire at a haunted theater, where Dr. Nolan and her boyfriend’s son are trapped,

The Angels Memorial doctors treat victims caught in a landslide, and a group of children come in from a camping trip with measles putting the entire ER at risk.

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 and Campbell are sent to a Russian submarine in U.S. waters to treat a sailor badly wounded during an explosion, and violent convicts injured during a prison riot descend upon Angels Memorial.

The doctors treat cult members who unwillingly survived a mass suicide attempt, and Mario decides to connect with his late father’s girlfriend.

Willis, Leanne and Jesse risk their lives to operate on an injured officer who has an explosive bullet lodged in her leg. Meanwhile, Mario, Noah and Guthrie treat a magician who comes to the hospital with breathing difficulties.

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.

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. As Campbell and Leanne divert high-risk patients to a nearby care center, Elliot and Heather are trapped in an elevator with a woman in labor, and Willis and Dr. Nolan try to help a 92-year-old woman who thinks it’s 1942.

Leanne defies Campbell's order and uses the hospital helicopter to reunite a family for the last time. Meanwhile, Willis and Loa use a cell phone app to communicate with an autistic young girl.

Guthrie becomes the hospital's new chaplain and considers undergoing a risky surgery to treat his Parkinson's. Meanwhile, the doctors treat patients involved in an apartment fire.

After pulling a double shift, Mario risks his own life when he follows Willis to a construction site where two brothers are trapped on a crane 300 feet above the ground.

The ER is quarantined when a viral outbreak jeopardizes the lives of the doctors and patients at Angels Memorial. Also, Ariel, the young girl Leanne bonded with a year ago when she lost her father, returns to Angels.

The doctors work with the Center for Disease Control and Prevention to find an antidote to the deadly viral outbreak at Angels Memorial. Also, Leanne makes a life-changing decision, and Jesse welcomes a new batch of residents.