The hospital remains in lockdown while the Center for Disease Control probes a mysterious and deadly illness that leaves Dr. Carter, Abby and others quarantined. While Dr. Lewis initially is left alone and fights to save one patient, Carter and Abby grow resolved to their romantic fate over the next several days as they idle away their time inside the hospital with Drs. Chen and Pratt -- and veteran patient Stan the Can. Meanwhile, newly widowed Dr. Corday struggles to adapt to her new medical practice in her native England while over-shadowed by her attentive parents.

Chaos reigns as the ER is overloaded with patients after another hospital is shut down just as Dr. Corday makes an emotional return -- and the aggressive Dr. Pratt enrages his counterparts when he resuscitates a heart attack victim who had been pronounced dead. As the tight-lipped Corday sees the ER for the first time since Dr. Greene's death, she clashes with the tightly wound Dr. Weaver. Meanwhile, Dr. Carter and a frisky Abby hardly care to conceal their growing romantic attraction to each other; Dr. Gallant gets an eye-opening education when he pulls a shift in the psychiatric unit; Dr. Romano faces a daunting new challenge.

After an enraged drug addict pulls a gun on Dr. Chen and other staff members, a fed-up Dr. Carter leads a divisive walkout when his request for metal detectors is delayed -- thereby endangering the lives of new patients. Elsewhere, Dr. Lewis counsels an exhausted elderly woman whose adult son is dying from Huntington’s disease -- the same affliction that killed her husband. In addition, Dr. Pratt tries to safeguard his developmentally disabled brother and comforts Chen after her brush with death; Dr. Kovac treats an HIV-infected boy whose parents arouse his curiosity; and Abby’s brother gets an eyeful when he visits the hospital.

Young Dr. Gallant gets worked up over the callous treatment afforded to an addled elderly patient under his repeated care and confronts Dr. Pratt when he reports a suicidal Army Ranger -- whom Gallant promised to protect. Likewise, a protective Dr. Carter is agitated when he discovers that girlfriend Abby downed alcohol while out with her girlfriends. Elsewhere: Dr. Weaver just isn’t herself as she makes repeated -- and embarrassing -- mistakes; Dr. Kovac dallies with the mother of a little girl facing kidney failure; and Dr. Corday treats the wife of a senior citizen who’s suffering from Alzheimer’s.

A suspicious fire on Halloween floods the E.R. with many bizarrely costumed victims -- frustrating Dr. Pratt -- while a frustrated Dr. Romano lets his recent traumatic injury interfere with good judgment when he directs Dr. Corday during surgery on a man with a flesh-eating infection. Elsewhere, Nurse Marquez threatens to file a grievance against Dr. Kovac, who must also treat an older mother who won’t accept that her newborn baby cannot survive while Abby and Dr. Carter playfully dress up in suggestive seasonal costumes. In addition, the staff greets an older but wiser surgical intern who tries to overcome a daunting physical handicap.

Dr. Corday clashes with the new surgical resident over his intervention in the treatment of a sickly young woman who signs a Do Not Resuscitate agreement while Abby wonders aloud about the erratic behavior of her visiting brother. Elsewhere, Abby feels torn when she is caught between an official nurse’s complaints against Dr. Kovac, her former lover. In addition, Dr. Pratt comforts Dr. Chen after she feels threatened by a patient and Corday suspects that an unconscious, 14-year old female patient has been sexually assaulted.

While Abby fears the worst when her AWOL brother suddenly disappears, Drs. Carter and Chen treat a frightened Chinese immigrant nanny whom they suspect was impregnated by her employer. Elsewhere, a new resident struggling to overcome his physical disability, tries to help in the E.R. -- but his advice to patients is unappreciated by his fellow doctors. In addition, Dr. Kovac cannot seem to quench his sexual appetites but his admirable resolve on the job to find an eye specialist to treat a young gunshot victim gets him into more trouble; Drs. Gallant and Harkins engage in an escalating game of one-upsmanship concerning their medical knowledge; Dr. Weaver gets some much-needed good news.

Abby's family nightmare continues when she and her mother go in search of her AWOL brother -- whom she suspects is suffering from serious mental problems -- and the two women meet at a Midwestern air force base in hopes of preventing his military court-martial. Back at the snowed-in hospital, Dr. Carter struggles to get out of Chicago to join Abby while Dr. Corday treats a family of patients who are severely struck by a truck driver while building a snowman. Meanwhile, Corday tries to help the father make a life-or-death decision while she sympathizes with the troubled Dr. Nathan as he struggles with his increasing symptoms of his physical handicap.

Abby is heartbroken when her troubled mother interferes with the treatment of her bi-polar brother at a psychiatric hospital while Dr. Pratt treats a frightened young girl who is injured in an auto accident and tries to hides a terrible secret. Elsewhere: a dissolute Dr. Kovac forms an unlikely bond with a lonely elderly woman who offers him a tidy tip; Dr. Chen is flooded with teary memories when a baby is abandoned inside the E.R.; and Pratt takes matters into his own hands when his luckless brother Leon is fired from his menial job.

In a form-breaking episode, events just before Christmas are traced in reverse chronological order as a hungover Dr. Kovac’s troubled life lurches closer to tragedy when his car accidentally hits another auto on icy roads -- endangering his passenger, young resident Harkins. While on duty, Kovac struggles to help a sickly young man fight for his life. In addition, Abby and the emergency room staff attend to a pair of belligerent elves and a young female hockey player with a head wound.

When Dr. Pratt is caught bringing a gun into the emergency room, Dr. Carter feels compelled to defend him -- but his colleague's erratic behavior makes even Carter have second thoughts. Meanwhile Dr. Weaver hopes to keep her pregnancy a secret, but physical complications might compromise her plans. Elsewhere, Dr. Lewis keeps a wary eye on a self-destructive patient who admits to molesting young boys; a guilt-stricken Dr. Kovac is warned by Dr. Romano not to admit his fault to the grieving relative of a patient he treated; Dr. Chen treats some student math whizzes who overdosed on Ritalin; and Dr. Gallant confronts a scheming young wife who has designs for her dying husband.

Dr. Weaver is caught in a bind when an influential alderman, seeking treatment after a fall, tests positive for syphilis -- information that will likely become public. Meanwhile, Dr. Carter is impressed when he meets an irascible old doctor who runs an inner-city medical clinic. Elsewhere: a sympathetic Dr. Kovac bends ethics when he tricks an injured patient into believing that his terminal wife has already expired; Dr. Pratt decides on his own to invite a dying mother's drug-addled son to her bedside and also keeps a wary eye on his mentally challenged brother Leon as he starts a new job; Weaver and Kovac differ on what to do with a young boy whose repeated medical history suggests paternal child abuse.

The protective Dr. Pratt is overwhelmed when his mentally challenged roommate Leon leads him to treat a gravely wounded suspect who was responsible for a robbery and holdup -- which also involved the easily duped Leon -- while Dr. Carter stuns Abby when he decides to go to Africa where the medical needs are greatest. Elsewhere, Carter attends to the sickly Dr. McNulty, the curmudgeon he recently met who runs a storefront medical clinic; the emotionally fragile Dr. Romano teeters on the brink of disaster and receives negative news regarding his recovery; Dr. Kovac intends to carry out a threat made to Dr. Weaver, who also re-encounters a grateful alderman who returns bearing financial gifts; and a skilled and bold doctor makes a pass at a shocked Dr. Corday.

Abby is distraught when she learns that her mentally troubled brother went missing while flying over the Great Lakes, a desperate Dr. Romano gets some depressing news and the hospital staff is confronted with a group of teenaged girls who are run down by a fellow female student suffering from a rare disease that might have caused the tragedy. Elsewhere, Dr. Chen’s relationship with Dr. Pratt takes a new turn; a daughter wants to take extreme measures to keep her extremely elderly mother alive; Dr. Weaver gets a promotion from a grateful alderman and Dr. Lewis ponders her pending blind date with a hunk.

As Dr. Carter returns from his scuba trip, Abby veers disturbingly close to a breakdown in her attempt to cope with a multitude of problems, Dr. Weaver confronts the man who stole her purse and Dr. Pratt accompanies paramedics -- and is later surprised by what happens to a comatose homeless man found on the street. As Abby awaits word on her missing flyboy brother and deals with her needy mom, she and Carter try to keep a brain-dead boy alive in order to comfort his grieving parents. Elsewhere, Dr. Kovac finally arrives at the hospital in the nick of time to aid an overdosed teen while Dr. Lewis helps an elderly man whose wife suffers from Alzheimer's and hypothermia. Lewis is also flattered by her teenaged admirer -- which doesn't help sugarcoat the grim news for her young cancer patient.

Drs. Pratt and Gallant see the when they are pulled over and questioned as possible suspects in a murderous rampage at the hospital's local diner while a sympathetic Dr. Lewis befriends an enamored teenaged cancer patient who fears he might not have long to live. Elsewhere, Dr. Carter learns a sobering earful from Maggie when he reluctantly takes her to the airport even as he ponders a life-changing personal decision. In addition, troubled Dr. Kovac ruminates on his tragic life when he is forced to consult a psychologist.

Dr. Weaver finds herself in a deep hole when she bends the rules to treat the syphilitic lover of her powerful new patron -- Alderman Bright -- but when the patient has a severe allergic reaction, she desperately tries to cover her tracks and compromises her ethics even further. In the meantime, Weaver's career continues to get a boost at the expense of a stubborn Dr. Romano and Dr. Carter and Abby reassess their relationship. Elsewhere: Dr. Kovac ruminates aloud on his star-crossed career path while he ministers to a rebellious teenaged girl who claims she was sexually abused at a boot camp for wayward youths, and Carter cares for a nasty middle-aged woman who's slowly being poisoned.

A tense E.R. spins further into chaos when a bitter Dr. Romano assumes control and offends nearly everyone with his inept and dictatorial policies, while Dr. Lewis is pleasantly surprised by the visit of an intimate relative. Meanwhile, a sympathetic Dr. Kovac employs Dr. Corday’s help in his quest to bring a sickly Croatian boy to America for his only chance to live. Corday also counsels a frightened young pregnant women whose cancer jeopardizes both her life and the baby’s. Elsewhere, Corday’s nanny accidentally runs over the foot of a cunning street salesman Dr. Pratt scrambles to buy Dr. Chen a gift for her birthday; and Lewis is startled at what happens to a woman who fell and is admitted with a broken wrist.

Abby is threatened by an over-medicated psychotic patient and gets an update regarding her missing brother. Elsewhere, Dr. Romano continues to make rash judgments -- and enemies -- when he orders a man with a severe sore throat to be released and then tries to perform surgery in the emergency room. Dr. Kovac welcomes a old friend -- a fellow Croatian doctor -- to observe the chaos of an American inner-city hospital. Carter also receives some emotional news from home as he guides a group of eager medical students who witness him refuse to give up trying to save a cardiac-arrested patient.

Kovac defies the rules to get a dying Croatian boy to Chicago in a race to save his life. Despite receiving tragic news, Carter tries to prevent grieving gang member Curtis from seeking vengeance. Carter’s sad family gathering is interrupted by Abby’s mentally troubled brother Eric, who has left his voluntary care facility. Meanwhile, Lewis has trouble ending her relationship with Chuck. Perez, a baseball player with a serious heart ailment, is treated on the eve of his pitching debut with the Chicago Cubs. And a tormented Romano makes a dramatic decision about his mangled arm, after it catches fire in his kitchen. Luka departs for the Congo after saying goodbye to Abby.

A frustrated Carter nearly has a meltdown when he treats two cult victims from a mass poisoning tied to the day’s unnerving solar eclipse as well as a wounded gang-banger, a dying Buddhist nun and a heart-attack victim. On the night scheduled to be his last in the E.R., Pratt treats a woman whose unborn child was cut out of her -- prompting a search for the baby -- plus a burned firefighter and other cult victims. Meanwhile, Dr. Romano experiences life as a patient when he undergoes surgery.

In a change-of-pace episode, a sweat-soaked Dr. Carter finally arrives in the war-torn Congo as part of a voluntary medical program and he finds primitive Third-World facilities, a few heroic staffers and woeful patients overflowing the tiny hospital while a bloody civil war threatens to engulf them all. Carter ventures deeper into the jungle to reunite with Dr. Kovac as they risk their lives manning a crude outpost used for a vaccination campaign -- and soon find themselves in the withering line of fire between the warring tribes.