Dr. Carter, Abby and other staff of the ER are quarantined as the CDC investigate a mysterious, deadly outbreak.

After another hospital shuts down, the ER is flooded with patients. Dr. Pratt resuscitates a heart-attack victim who had been pronounced dead.

Dr. Carter leads a walk-out of staff protesting lack of security in the ER.

Weaver makes a mistake on live television while demonstrating flu shots, inadvertently sticking two reporters with the same needle. In the bathroom later, Susan and Abby catch Weaver in a stall with a needle. Weaver confesses to Susan that she's taking hormones. Carter, assuming Susan knows already, tells her about Abby's drinking problem. Abby becomes nurse manager, and argues with Carter about her drinking. Luka flirts with a patient's mom and is caught emerging from a storage closet with her. Gallant has a rough day: Pratt butts in on a patient of his, and Stella Willis dies because Kayson didn't bother with thorough treatment.

On a busy Halloween night, Carter and Abby are the only staffers in costume when victims of an apartment building fire pack the ER. Corday takes on a new med student for a surgical rotation - Paul Nathan, who suffers from Parkinson's disease and decided to pursue medicine after being diagnosed. Despite a difficult first day, both professionally and medically, Nathan makes a difficult diagnosis. Romano's rehab lags behind his expectations, and he begins to doubt his role in surgery is no longer an option.

Corday clashes with new surgical resident Paul Nathan regarding his intervention into the treatment of a sickly young woman, Alison, who has signed a "do not resuscitate" order. Abby wonders about the erratic behavior of her visiting brother Eric, and feels conflicted over a nurse's official complaint against Kovac, Abby's former lover. Meanwhile, Pratt comforts Chen after a patient threatens her, and Corday suspects that an unconscious, 14-year-old female patient has been sexually assaulted.

Abby fears the worst when her brother Eric suddenly disappears. Carter and Chen treat a frightened Chinese immigrant nanny, Tong-Yi, whom they suspect was impregnated by her employer. Meanwhile, new surgical resident Paul Nathan, who is struggling to overcome a physical disability, tries to help in the emergency room—but his fellow doctors do not appreciate his advice to patients. Unable to quench his sexual appetites, Kovac also gets into trouble in his professional life, despite his admirable resolve to find an eye specialist to treat a young gunshot victim. Gallant and fellow medical student Erin Harkins engage in an escalating competition of medical knowledge. And Weaver gets some much-needed good news.

Abby's family nightmare continues as she and her mother Maggie search for Abby's missing brother Eric, who Abby suspects is suffering from serious mental problems. Meanwhile, Corday treats a family of patients who, while building a snowman, were severely injured after being struck by a truck driver.

Abby is distressed when her mother interferes with Eric's treatment at a psychiatric hospital. Pratt treats a frightened young girl who has been injured in an auto accident and desperately wants to hide a secret. An unhappy Luka forms an unlikely bond with a lonely elderly woman who makes him a surprising offer. Chen is overcome with upsetting memories when a baby is abandoned in the emergency room. Pratt takes action after Leon is fired from his menial job.

Dr. Kovac's speeding causes a crash on an icy road. The ER staff treats angry elves and a female hockey player with a head injury.

Dr. Pratt brings a gun into the ER; Dr. Weaver tries to conceal her pregnancy; a woman plots against her dying husband; Dr. Chen treats math students who overdosed on an unknown stimulant.

Dr. Weaver is in a difficult situation when an alderman tests positive for syphilis. Dr. Carter is impressed when he meets a man who runs an inner city clinic.

Dr. Pratt's disabled roommate leads him to a wounded thief. Dr Carter decides to practice medicine in Africa. A doctor makes a pass at Dr. Corday.

Abby comes close to breaking down trying to deal with her mother. Dr. Pratt goes for his required paramedic ride-along. Dr. Weaver confronts the man who stole her purse. Dr. Lewis has a teen admirer.

The police question Drs. Pratt and Gallant about a rampage at the diner across from the ER. Dr. Lewis befriends her teen admirer, a cancer patient.

Dr. Weaver bends the rules and treats the Alderman's lover for syphilis. Dr. Kovac's patient claims she was sexually abused at a boot camp for troubled youth. Dr. Carter treats a women with a nasty personality.

Dr. Romano seizes control of the ER and manages to offend everyone. Dr. Kovac asks Dr. Corday's help in the quest to bring a sickly Croatian boy to the U.S. for medical treatment.

A psychotic patient threatens Abby. Dr. Weaver suspects a woman poisoned her own baby. Dr. Romano continues to make rash judgments.

Dr. Carter tries to prevent a grieving gang member from going after vengeance. Dr. Kovac races against time to save a Croatian boy's life. A baseball player gets treatment for a heart problem.

Carter's day shift and Pratt's night shift (his final shift at County) are intercut. Both men have extremely busy days. Doc Magoo's burns down. Luka calls Carter from the Congo and asks for help. Carter decides to go. Pratt decides to stay at County. In other parts of the hospital, Romano has his now-useless left arm amputated.

Carter travels to the Congo after Luka calls and asks for help. During the course of his two weeks there, he learns how much different the conditions are against what he's used to. Luka decides to stay for a while as Carter heads back to Chicago, ending up at a sleeping Abby's bedside.