Season Premiere: Multiple traumas come to the ER after a truck crashes into a coffee bar, narrowly missing a very pregnant Carol. Romano becomes Chief of Staff when Anspaugh announces his retirement. Weaver becomes even more unpopular when she is appointed permanent Chief of the ER by Romano. When she becomes engaged to Reggie, Jeanie decides she wants to adopt an HIV-positive baby. Two new doctors are introduced: Luka Kovac, a Croatian doctor moonlighting in the ER; and Cleo Finch, a pediatric resident in the ER. The ER is thrown into chaos when a patient pulls the fire alarm.

Returning from his mother's funeral, Mark clashes with Kerry over the treatment of a teenager who rescinded her DNR. A new second year resident that likes to be called "Dr. Dave" clashes with almost everybody from the surgical department to the paramedics. Romano promotes Corday to Associate Chief of Surgery. Jeanie decides to marry Reggie after her bid for adoption is denied. Carol and Luka treat a dying woman who wants to see a priest. Peter and Carla meet with a psychologist about Reese. Carla later tells Peter that Reese may not even be his son, an idea he vehemently denies.

Mark clashes with new attending physician Gabriel Lawrence. Lucy and Luka try to convince a woman that her husband is dangerous. Carter tries to be supportive as Elaine undergoes her mastectomy. After Weaver puts in a good word for her, Jeanie receives baby Carlos on a temporary basis; she and Reggie get married later in the day. Cleo treats a family who was in an accident. Benton talks with his lawyer about having a DNA test. Mark and Elizabeth spend their first night together.

Lawrence exhibits signs of forgetfulness, which Lucy catches, while treating a teenager who tried to hang himself. A negative article about the hospital appears, following a tour and interview given by Corday. Malucci learns a lesson in sympathy dealing with a man dying from a skydiving accident. Cleo is racked with guilt after misdiagnosing a four-year-old with iron poisoning, who later dies. Carter makes plans to meet with Elaine. Carol treats a pregnant waitress with no insurance.

An explosion in a high school science lab brings a flood of patients to the ER. Lawrence's memory continues to deteriorate, and more staff members become suspicious. Elaine tells Carter that she intends to go to Europe for a couple of months. Lucy and Carol compete for the one available rehab bed each of their patients needs. Dr. Dave learns about sympathy from a girl whose face was burned in the explosion. Benton inquires about DNA testing. Jeanie's home life becomes hectic, taking care of her new baby.

Benton struggles with taking a DNA test to determine whether or not Reese is actually his son. As the Lawrence's performance deteriorates, Kerry and Mark use a fake patient to convince him of his Alzheimer's. Carter has trouble persuading the victim of a nursing home fire to sign a DNR for her husband and later is asked to fake a diagnosis for a family with no insurance. Carol continues to meddle in Meg's life, having her arrested so that she can be detoxed. Cleo is blamed for an explosion in the suture room, that might actually have been Dr. Dave's fault.

Greene searches for his father, who is supposed to have arrived from San Diego earlier in the day. Corday crosses ethical lines to save the life of a rapist while Kovac and Weaver work feverishly to save the woman he raped. Lucy regrets letting a choir director leave the hospital. Cleo treats an alcoholic teenager. Dr. Lawrence saves a life before departing to live with his son. Weaver hires Kovac as the ER's new attending. Carol visits Meg, who has now had her son and is preparing to go to jail.

Carol goes into labor on the El train. Luka finds her and helps her to the ER. She delivers one twin there and is then taken up to Maternity, where she is cared for by OB nurse Abby Lockhart. Dr. Coburn delivers the second twin by emergency c-section. Mark abandons his Thanksgiving dinner at Elizabeth's to be Carol's birthing coach, leaving Elizabeth with his grumpy father and a rather sullen Rachel, who unexpectedly experiences her first menses. in the ER, Luka cares for an old woman who is dying on what happens to be her birthday. Dr. Dave helps Carter with a diagnosis of a very rare disease, and reveals that he went to medical school in Grenada.

Lucy desperately tries to find a way for a young woman to receive a new heart in order to stay alive, going to Romano's house in the middle of the night to find the only doctor qualified to perform the surgery. Corday tries to convince Dean Rollins to have surgery on his leg. Carter gives away the ER's Secret Santa gifts as part of aballoons-for-toys exchange. When Kate comes down with a fever, Carol takes the twins to the ER, where Luka takes care of them. Alcoholic teenager Chad Kottmeier returns; Cleo enters him in an alcohol treatment facility, taking him away from his mother. Carla tells Benton that Reese will be staying in Chicago because her husband lost his job transfer.

Greene flies to San Diego when his father runs away from his retirement home. Former med student Deb Chen returns to the ER, now known as Jing-Mei Chen, and also a full-fledged doctor. On her first day, she and Carter help a young Jane Doe brought in for panhandling. Corday tries to coerce Dean Rollins into revealing the location of another body. He asks her to end his suffering. Kovac tries to help two brothers, one of whom is mentally-challenged. Finch treats a young teenager who may have too much on her plate. Dr. Dave tries to fix Weaver's car, and has to break bad news to a young kid.

Lucy struggles to get patient Valerie in for surgery when an infection threatens her heart transplant. Carter and Chen disagree over the treatment of an elderly woman and her care-giving daughter. Mark tries to help an abusive gay couple. Carol has trouble adjusting after her return from maternity leave. Dr. Dave goes out of his way to shut an illegal pharmacy down. Luka treats some circus performers. The ER suffers from a water shortage. Benton asks Cleo out.

Third year med student (and part-time OB nurse) Abby Lockhart begins her ER rotation. A flu bug sweeps through the ER. Carter tries to help a kid with muscular dystrophy while Cleo treats a kid who's been injecting hormones into himself. Corday discovers that her mother will be coming to town and then decides to pay her a visit. Lucy treats a dying homeless man. Benton and Romano clash over the treatment of a surgical case. Luka and Carol treat a young boy whose mother is injecting him with saline to get surgical procedures done.

It's Valentine's Day in the ER. However it proves to be a tough day when Chen and Malucci clash over the treatment of a patient, the parents of two young children die in a car accident, Lucy and Carter treat a law student who she believes may have mental problems, Romano and Elizabeth operate... on his dog, and Abby learns that not all ER cases have happy endings when she treats a dying elderly woman. Mark and Elizabeth go out on a double date with her mother and his father. At the end of the day, the staff party, unaware that Carter and Lucy have both been cuddled by Lucy's patient.

Carter and Lucy are found almost bleeding to passing by Weaver. The two are worked on in trauma and then are operated on up in Surgery. Lucy's injuries appear serious when her chest needs to be cracked open. Because of Anspaugh's quick thinking, Carter survives, however, Corday and Romano are unable to save Lucy, which throws the ER into devastation. Cleo is forced to crack open a man's chest to save his live when no surgeon is available, much to Benton's dismay. Lucy and Carter's attacker is found after he is hit by a car, and he is admitted to Psych with schizophrenia.

Corday discovers that her mother and David Greene spent the night together. Greene later discovers that his father has lung cancer. Lucy's mother visits the ER to empty her daughter's locker and pay a visit to Carter, who is regaining strength and recovering. Abby goes behind Weaver's back with a patient on a gut instinct. Luka witnesses a hit-and-run accident and rushes the victim to the ER in a plumber's truck. Carol must tell a promiscuous, sexually active 14-year-old that she has cervical cancer.

Greene takes charge of a chaotic shift while dealing with his father's refusal of treatment for his lung cancer and Corday's insistence on harvesting a brain-dead patient's organs. Carter returns to work and later visits his grandmother. Carol deals with constant phone calls from her nanny. Abby commits an error with a patient, costing him his life. Dr. Dave eats cereal with Carol's breast milk from an emesis basin.

Mark and Elizabeth squabble after he disagrees with her treatment of his father. Carter gets in trouble when a patient that he had discharged for insurance reasons comes back into the hospital near passing. Weaver is suspended after disobeying a direct order from Romano, regarding the care of a mentally challenged girl. Kovac and Corday are faced with a blitz of possible kidney receivers, but Luka has a problem with the one who gets it. Cleo and Benton endure Romano's ire for scribing a messy prescription that nearlyhugs a man; they later have sex on the stairs at her place. Chen faces some personal obstacles when treating a man who refuses to tell his daughter about his genetic condition.

Greene deals with his father's worsening condition and his desire to be placed in a hospice. Carter almost loses it as he treats a construction worker he thinks is faking an injury. Carol's girls start daycare and she reveals to Mark that Doug has asked her to move to Seattle again. Cleo asks Benton to overrule Corday's surgical advice concerning a teenager who was in an auto accident. Romano takes control of the ER in the wake of Weaver's suspension. Kovac treats a man having anxiety attacks from his impending divorce. Abby treats a mom with five kids who is pregnant again and considering an abortion. The ER gets a letter for Lucy, informing her that she had matched the County General psych department.

Greene takes the day off to care for his dying father but ends up in the ER anyway where the older Greene is diagnosed with pneumonia. Carter is plagued with flashbacks from his attack and visits Samantha Sobricki in an attempt to gain some insight on why Paul might have gone off the handle. Carol and Luka spend a day on the town shopping for a used car, and are drawn into a kiss. Weaver returns from her suspension. Abby treats a young girl in need of a bone marrow transplant, but whose mother won't allow it out of spite for the father. Benton and Finch have breakfast with his sister, where Jackie tells Peter that Cleo is not serious about him.

Carter buries himself in his work to take his mind of his troubles, coming under fire from Weaver and Kovac when he defies ER policy and performs an ICU procedure himself. Luka, meanwhile, has his own problems when Benton confronts him about poor patient follow-up. He also attempts to romance Carol, who is distracted from her work by the arrival of a birthday present from Doug. Chen lobbies Romano to do genetic testing on all newborns and is forced to blackmail him into signing a petition for it. Corday tries to help an anorexic girl determined to leave the hospital. Malucci discovers that an accident victim has been sexually abusing his six-year-old daughter. David Greene dies in his sleep, but not before giving Corday his late wife's pearl necklace and telling his son he is proud of him.

When Carol resuscitates a terminal cancer DNR so that she can say goodbye to her family, she realizes that life is too short; after saying a quick goodbye to Luka and Mark, she runs out of the hospital and hops a plane to Seattle to see Doug. Carter's mental stability has deteriorated to the point where he almosthugs a woman. Chen notices and tells Mark her concern that Carter is bipolar. Weaver goes on the warpath after noticing staff members taking shortcuts in their work, including Abby, who discharges a woman prematurely without running extra tests. Corday offers Malucci a sobering assessment of his performance. Mark prepares to bury his father, and enjoys his daughter's company.

Season Finale. Kovac and Benton are rushed to the site of a school shooting where they disagree over who is more critical: the injuredballoonman or one of his victims. Kovac later treats a heavily pregnant teenager who didn't know she was pregnant and refuses a C-section to save the baby's life. After Abby catches Carter injecting a patient's leftover pain medication into his wrist, Kerry, Mark, Benton, Chen and Anspaugh try to convince him to seek professional help.