ER explores the inner workings of an urban teaching hospital and the critical issues faced by the dedicated physicians and staff of its overburdened emergency room.
Weaver returns to the emergency room after a self-imposed sabbatical of three weeks, wondering how her newfound lesbian identity will affect her workplace relationships. Having purposely allowed a mass murderer to die, Greene must keep the secret amidst the scrutiny of his superiors.
Carter is overwhelmed by patients from a rock concert riot. Benton's former girlfriend is killed in a car accident. Newly promoted to chief resident, Chen panics and misdiagnoses a patient. Kovac angers Abby by flirting with a waitress (JULIE DELPY).
Greene's daughter comes to live with him and Corday. Carter and Abby become more intimate. Chen must prove her skills while saving a baby. Weaver terminates Malucci after witnessing him having sex in an ambulance.
Dr. Susan Lewis arrives, looking for work in the Chicago area again. Greene hires her, over Weaver's objection. Chen nervously attends a risk management review, accompanied by the unforgiving Weaver, to discuss a patient who recently died under Chen's supervision.
Lewis faces a hectic first day back at County General. Benton and Roger become frantic when they can't locate Reese. Carter's grandmother is brought to the emergency room. Carter and Abby deal with inept medical students.
Corday's ordeal, caused by the recent infection-related deaths of several of her patients, worsens when she is closely observed by a stern taskmaster. Corday is outraged when a county investigator accuses her of deliberately euthanizing the patients, who all happened to be elderly.
Corday desperately searches for whoever is secretly murdering her elderly patients, implicating her. Greene must deal with overstressed Corday and unruly Rachel (recurring guest star HALLEE HIRSH), who is in trouble at school. Benton reluctantly agrees to a paternity test to prove he's really Reese's father.
When a torrential rainstorm pounds Chicago, patients deluge the emergency room while Weaver speeds to an accident site. There, she finds a pregnant woman, Vicki lying injured inside a wrecked ambulance while live power lines spark. Fourth-year student Michael Gallant makes a dangerous decision in order to help.
Benton and his attorney, Janice Prager (recurring guest star ROMA MAFFIA - "Profiler"), begin an arduous court fight for Benton to take custody of his son from Roger (recurring guest star VONDIE CURTIS-HALL). Corday waits for a distraught husband to decide if surgeons can remove his fatally injured wife's organs for transplant before it's too late.
As Christmas nears, Benton endures bouts of rage and frustration when the custody trial for his son, Reese (recurring guest star MATTHEW WATKINS), reaches a critical stage. Benton must choose between his job and his child unless cranky Romano agrees to tailor Benton's work schedule.
On her birthday, Abby receives a few unwelcome surprises, including the news that Kovac plans to leave town for two months to work in Bosnia. She also finds out that her ex-husband is engaged to remarry. Despite Abby's efforts to prevent it, Carter has an uncomfortable encounter with the patient, Paul Sobriki who stabbed him and killed a fellow doctor two years ago.
Lewis clashes with Weaver over the treatment of an injured condemned murderer, Mike Kinney (CYRIL O'REILLY), who wants to die in the emergency room rather than wait for his execution. Abby urges her battered next-door neighbor, Joyce (recurring guest star CHRISTINA HENDRICKS), to seek shelter from Joyce's abusive husband, Brian (recurring guest star MATTHEW SETTLE).
Greene and Corday are terrified when their helpless baby daughter is brought in, suffering from an accidental overdose. Lewis treats a father and daughter who were injured after opening a letter bomb. Weaver seethes when Romano gives some of her numerous duties to Lewis.
With their baby daughter still fighting for her life, Greene and Corday argue after she orders Rachel out of their home. Abby regrets championing her battered neighbor, Joyce who refuses to charge her husband, Brian with abuse. Tainted snacks cause food poisoning among several staffers.
As Greene's medical problems reappear, Corday lives in a hotel room with the baby, angry at Greene for the way he dealt with Rachel (recurring guest star HALLEE HIRSH) following the infant's overdose. Greene treats a father, Ian Nevinger (BRANT COTTON), who severely burned himself while experimenting with illegal drugs.
After a dominatrix, Medusa (PATRICIA THIELEMANN), inadvertently leaves a bag of sex toys in the emergency room, causing a few moments of hilarious but inappropriate behavior amongst the overworked staff, Weaver angrily orders Abby, Carter, Lewis, Kovac and Gallant to attend a day-long sexual harassment seminar.
Still living apart from each other, Corday and Greene find it awkward to work together in the emergency room. Corday suspects that Greene is romantically involved with Lewis. Greene is confused by the reaction of Terry, the sister of a severely ill woman, Joanne, after he asks her to donate part of her liver to save Joanne's life.
While Greene tries to distract himself from his worsening brain tumor by working hard, he reluctantly recognizes that his debilitating illness is hampering his ability to perform the job. Watching other terminally ill patients struggle with their treatments also has a sobering effect on him. Greene has a poignant reunion with his ex-wife, Jenn, and mediates between the staff and a stubborn but deluded homeless man, Al Ervin.
Extremely disturbed after receiving a traumatic call for help from her 6-year-old niece, Lewis rushes from Chicago to New York to search for the child. Lewis tracks down her trouble-prone sister, Chloe , and enlists the aid of police officers Faith Yokas and Maurice "Bosco" Boscorelli and paramedics Kim Zambrano ("Third Watch" series star KIM RAVER) and Alex Taylor.
Carter reads a letter from the ailing Greene to the emergency room staff and feels helpless when a terminally ill homeless man, Al Ervin (PAUL BENJAMIN), keeps requesting Greene's help. Carter also tries to coax a drunken Abby to attend an Alcoholics Anonymous meeting with him. Meanwhile, Weaver berates a psychiatric patient.
As Greene lives his final days, he takes the reluctant Rachel (recurring guest star HALLEE HIRSH) to his childhood home in Hawaii, along with Corday. There, he indulges in life's simple pleasures and tries to impart memories, some of which are bittersweet, and family history to his distant daughter.
In the season finale, Chicago residents become fearful of an epidemic. When Carter and Abby treat two children with ominous symptoms, the emergency room is placed under quarantine until disease experts from the federal government can arrive. As Carter fights to save the youths, the staff is trapped with the suffering patients and a busload of injured and irritable senior citizens.