Multiple traumas come to the ER after a truck crashes into a coffee bar, narrowly missing a very pregnant Carol. Rumors fly around the ER about Anspaugh retiring as chief of staff and Romano taking his place; the rumors are later found true. Mark speaks out against Romano in a staff meeting. Weaver double-crosses him to move ahead with her career and is later named permanent chief of the ER. Carter meets his cousin's ex-wife, whom he is attracted to. Reggie proposes to Jeanie, and she decides to adopt an HIV-positive baby. Benton and Carla meet with a mediator. Lucy asks Weaver to mentor her; Weaver turns her down because she is the new chief and it would be inappropriate. Two new doctors are introduced: Dr. Luka Kovac, a Croatian doctor moonlighting in the ER, and Dr. Cleo Finch, a pediatric resident in the ER.

Returning from his mother's funeral, Mark clashes with Kerry over the treatment of a teenager who rescinded her DNR.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.

Lawrence exhibits signs of forgetfulness, which Lucy catches, while treating a teenager who tried to hang himself. A negative article about the hospital and Corday appears in the newspaper after last week's reporter visit. 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 many patients to the ER. Lawrence's memory continues to go downhill and more staff becomes suspicious. Elaine tells Carter her intentions to leave for Europe for a couple of months and they cuddle. Lucy and Carol compete for one rehab bed for each of two patients--a painter snorting coke and Meg, the pregnant waitress, who is shooting heroin. Dr. Dave learns some sympathy from a girl facially burned in the explosion. Benton inquires about DNA testing. Jeanie's home life becomes hectic, taking care of her new baby.

Benton struggles with the idea of taking a DNA test to determine whether or not Reese is actually his son. As the signs continue to pile up, Kerry and Mark use a fake patient to convince Dr. Lawrence of his Alzheimer's. Carter has trouble persuading a woman from a nursing home fire to sign a DNR for her husband and later is asked to forge charts 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, but it may actually be 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. Carol visits Meg, who has now had her son and is preparing to go to jail.

Carol goes into labor on the L. With the help of Luka, she makes it into the ER and delivers one twin, Tess, there. She is then moved to the OB and put in the care of OB nurse Abby Lockhart. Dr. Coburn delivers Kate through an emergency c-section but Carol and both babies turn out to be fine. When Mark has to leave Thanksgiving at her house to be Carol's Lamaze coach, Corday is stuck with his father and Rachel. Dr. Dave helps Carter with a diagnosis of a very rare disease.

Lucy desperately tries to find a way for a young woman to receive a new heart in order to stay alive, going as far as 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 a guns-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. 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.

The staff prepares for Valentine's Day. Romano brings his dog into the OR for surgery. The rivalry between Chen and Malucci hurts a patient of theirs. Abby treats a former patient of Carter's and learns that not every story in the ER has a happy ending. Luka must tell two children that both of their parents are dead. David Greene and Isabelle Corday meet and hit it off. Lucy seeks advice from Carter while treating a young law student who may have schizophrenia, but he keeps brushing her off. When he finally gets around to checking on Lucy, he finds her lying on the floor, stabbed in the chest, just after being stabbed himself…

Weaver discovers Carter and Lucy. As police begin to arrive, Benton rushes between the two, being the only surgical resident on duty. Greene and Corday are paged to return to the hospital. Carter almost loses his kidney, but with quick thinking by Anspaugh, Benton is able to save it and Carter will be okay. On the other hand, Lucy has her chest cracked because her heart stops but she is stabilized. In the OR waiting room, while waiting for emergency surgery to fix a life-threatening blood clot, Lucy flatlines. Romano and Corday desperately try to save her life but she dies and the staff receives the news. Meanwhile, Paul Sobriki is found running naked down Michigan Avenue and is brought in. His wife arrives at the ER; incapable of believing her husband would be capable of such a violent act. And while Benton is busy with Carter, Cleo is forced to crack a man's chest open to save his life, resulting in Benton's ire.

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 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 drinks Carol's breast milk with Corn Pops in a vomit bowl.

Mark and Elizabeth squabble after he disagrees with her treatment of his father. Carter gets in trouble when a patient of his that he had discharged for insurance reasons comes back into the hospital near death. 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 nearly kills a man. 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.

Carol resuscitates a terminal cancer DNR to say goodbye to her family, and realizes that life is too short. She runs out of the hospital after saying a quick goodbye to Luka and Mark and hops on a plane to Seattle to see Doug. Carter's mental status is dissipating rapidly to the point where he almost kills a woman. Chen notices and tells Mark her concern that Carter is bipolar. Weaver goes on the warpath after noticing ER staff 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.

Kovac and Benton are rushed to the site of a school shooting, where they disagree over who is more critical: the severely injured gunman or one of his victims. Luka later tries to convince a stabbing victim to have surgery to save her unborn child. Abby catches Carter injecting unauthorized pain medication into his wrist. She tells Greene, who confronts Carter with Weaver. Carter denies it but Greene takes him off patient care, assigning him paperwork. Greene stages an intervention with Chen, Benton, Anspaugh and Weaver, giving Carter two options: he either goes to Atlanta and a detox facility, or he is fired. Carter quits and walks out of the hospital. Benton confronts him with a reminder of what happened to Carter's cousin Chase. Carter hits him but then breaks down in his arms. Later, as Weaver waits for word that Carter has gotten on his way to Atlanta, we see Benton sitting silently next to him on the plane.