Continuing where last season's finale left off, Carter returns from Africa, going straight to Abby's bedside. Her response to seeing him is to ask for her key back. She is very chilly towards him all day until new med student Neela Rasgotra answers a phone call for Carter. The message: Luka is dead. Carter decides to go back to the Congo to bring the body back. Abby pleads with him to stay, but he leaves anyway. Neela has a tough first day in the ER, earning Chen's ire after Pratt flirts with her. Gallant, now a full-fledged doctor, helps to show her the ropes though. Romano returns from his medical leave. Susan is still seeing Chuck. Weaver screams a lot.

Carter reunites in the Congo with Charles and Gillian. Through flashbacks, we see Luka after Carter left 22 days before. He has caught malaria, and is soon captured by Mai Mai, who begin to murder the people he's with. Gillian introduces Carter to Debbie, a worker for the Red Cross. They search for Luka, and when finally shown the body, it's not even him. It turns out that a cross given to Luka by one of his patients led the Mai Mai to believe he was a priest. Carter, Debbie and Gillian find Luka, nearly dead, and put him on a plane back to Chicago. Carter gives him a letter to give to Abby and the news that he's going to stick around in the Congo for a while.

New residents arrive at County, including 2nd year Nick "Coop" Cooper. Coop talks back to Romano and is nearly killed when Neela gives him too much epi during an asthma attack. Pratt and Chen's dinner is invaded by her parents. Chen later dumps him when he states his feelings about their relationship. Romano's hospital insurance doesn't cover a good prosthetic arm. Instead, he gets a hook. He has been trying to cut the budget by giving the nurses fewer shifts and filling the rest with temps. Most of the nurses stage a walk out and receive 90-day suspensions as a result. Abby and Lewis treat a girl whose parents don't want to tell her that she's dying. Fast Eddie and Corday do it in a car. Luka arrives at County and Gillian gives Abby her letter from Carter...in which he dumps her. Frank picks it up off the ground from next to a trash can and soon everybody knows.

Pratt is left in charge of a busy night shift, with only a few med students and fellow 2nd year Dr. Coop. Both Neela and Pratt make mistakes with their patients. Abby is looking for a loan for a secret endeavor; she finally gives in and begs her ex-husband to co-sign. Romano gets his Utah arm. Abby visits Luka, finding Gillian in the middle of saying goodbye. Corday and Dorset are still doing it, although Corday wonders if it's too soon after Mark's death.

Luka's first day back in the ER is a busy one. While working out the details of his return with Weaver, he mentions that he'll be returning to the Congo in two months. Lewis treats an alcoholic mom who sets herself on fire in front of her son, and an elderly architect who tried to commit suicide because he's going blind. Abby appears as a surgical consult; she's back in med school. Sam Taggart replaces her on the nursing staff. Dorset's wife calls while he's in surgery with Corday.

Since his return to County, Luka has become obsessed with treating and streeting patients as quickly as possible. He argues with Pratt over the treatment of a pregnant woman who doesn't want her premature baby, as well as with Romano, who wants more tests ordered on patients so that the hospital can make some money. Abby is still on her surgical rotation and Romano is still riding her hard. Coop flirts with Chen. Susan has been reading to Ben Hollander in her spare time; he seems to have developed a crush on her. Luka meets Sam's son Alex in the new JumboMart (which has replaced the fire-damaged Doc Magoo's).

Lewis has a rough day: Chuck proposes having kids with her, she misses an appointment with an auditor, and Ben Hollander commits suicide. Pratt receives an unfavorable review from Romano and also treats a con who was raped in prison. Abby, on her ER rotation, agrees to study with Neela. Gallant makes a decision that results in the death of a young girl with leukemia. Sam's son Alex spends the day in the ER, following Luka around for most of it. Chen gets a call from the Chinese embassy and heads for a plane: her vacationing parents were in an accident.

A helicopter crashes on the roof, falling into the ambulance bay where it lands on Romano and causes a huge explosion. Pratt and Morris are pretty lucky: Romano had just fired Pratt and caught Morris smoking weed in the ambulance bay. Susan worries that Chuck was onboard, although it turns out he wasn't, even though he does have some internal injuries. He'll be fine though. Abby daydreams about what Carter's up to in Africa. Luka enjoys Thanksgiving dinner with Sam and her son Alex.

Gallant's sister is in town and Pratt hits it off with her. Nobody attends Romano's memorial service except for Corday. Alex steals a severed finger and later goes missing, turning up at County to see Luka. Luka and Sam share a moment. Neela treats two Amish kids who were injured on a sojourn into the city.

What has Carter been up to for the past seven months? He's been in the Congo developing a relationship with Makemba "Kem" Likasu, an AIDS-clinic worker, who is six weeks pregnant with his child. Carter needs to return to Chicago to take care of his grandmother's estate, so he talks Kem into going with him to have the baby in Chicago.

Carter is back in Chicago with Kem in tow. His return to the hospital is uneventful, although there is an awkward moment when Kem meets Abby. Sam gets called to Alex's school: it seems that he was suturing his own cut. She asks Abby whether the nurses' insurance covers family therapy. Gallant discovers that his sister has been getting it on with Pratt, although she later dumps him. Pratt accidentally breaks a patient's neck while doing an unsupervised intubation. Missing Valium is found in Weaver's labcoat, although it's implied that Morris took it. Luka informs Weaver that he's leaving for the Congo in two weeks. However, he later decides to stay in Chicago indefinitely.

Abby and Neela spend 21 days on their NICU rotation. Life in the NICU is certainly not fun, what with the sick babies and all. Parents of twins return to China with only their healthy son. Another couple spends 21 days in the NICU before finding out their son will die. Abby and Carter discuss his impending fatherhood. Weaver and Sandy have a baby boy.

Kem accompanies Carter to work and manages to annoy about half the hospital over the course of the day. Weaver has used the money that Romano left to the hospital and unveils the Robert Romano Center for Gay, Lesbian, Bi and Transgendered Health Care. Gallant is doing a ride-along with Morales and Bardelli. Weaver offers Lewis the acting ER chief job, but Susan refuses because she's pregnant. Pratt is questioning his abilities as a doctor. Chen calls the ER to tell Pratt that her mom has died in China. Sam finds out about Luka's reputation as the ER bed hopper, but finds herself making out with him by the end of the day.

Sam treats a mother whose family was in a car accident and a 16-year-old pregnant girl whose boyfriend loaned her out to his friends. With Alex at a sleepover, she gets lonely and heads over to Luka's, where they consummate their relationship. Carter shows Kem the city and proposes they move into a townhouse he bought for them, but she leaves to go back to Africa for a while. A Human Resources consultant sent to the ER observes the staff.

An entire family suffering from carbon monoxide poisoning is brought to the ER. Claustrophobic Neela must spend a couple of hours in a hyperbaric chamber with the newborn baby. Carter misses Kem, who's been gone for a month now, so Jerry arranges a webcam chat for them. Luka and Sam have been keeping their relationship a secret from Alex, although he finds out and isn't too thrilled about it. After Sam suggests they take their relationship slower, Luka isn't too thrilled either. Dr. Lawson asks Elizabeth out. Sandy's mom brings Weaver's son Henry in. Chen returns from China: her dad needs 24 hour nursing care and she's having trouble getting adjusted to that idea. Susan and Chuck visit her dad and tell him about the pregnancy.

Morris streets a patient without getting a psych consult, and the guy steals a tank and heads for County. Gillian is visiting Luka, and Sam's not happy about it. Luka and Sam make up by the end of the day though. A 13-year-old boy has chlamydia. Frank has a heart attack. Corday plays the dating game, going out with two men.

Carter sticks up for Neela when the other attendings doubt her future in the ER. Neela caves to the pressure and accidentally gives a patient the wrong medication. Gallant takes the blame for her though. Sam and Luka try to plan Alex's birthday party. Frank visits on his way to therapy. Chen's home nurse quits after her dad behaves poorly.

Sandy Lopez dies after going into a fire in an abandoned warehouse. Kerry is devastated; even more so when Sandy's family won't let her see Henry. Susan and a guy from Risk Management interview everybody who was involved in last week's death. Gallant reveals that the Army is sending him to Iraq and he and Neela share a bittersweet farewell. Sam is surprised by the arrival of Alex's dad. Neela passed her boards, although Abby did not. Corday discovers that she's the new Head of Surgery. After she has contractions, Susan is put on bed rest for the rest of her pregnancy--five weeks.

A patient accuses Pratt of paying too much attention while giving her a breast exam. Luka seems okay with Alex's father hanging around. Abby's on her psych rotation, while Neela begins her research rotation. Chen's dad is admitted to the hospital. Weaver is granted custody of Henry for the weekend. Corday is uncomfortable when Dave and Dr. Lawson meet. Abby discovers that Carter paid her med school tuition.

Still on her psych rotation, Abby treats a young mother who's having seizures caused by bad memories of her young son who died eight years earlier. Later, she takes her boards again. Weaver considers leaving town with Henry. Neela saves the life of one of her fellow researchers. Kem returns to Chicago, just in time for a Carter Foundation board meeting. Carter announces that the business will be switching focuses from arts & entertainment to medicine. His father and other board members are not pleased.

Kem goes into labor, but she and Carter are shocked to discover that the baby has no heartbeat, and will be stillborn. 19 members of Neela's family are coming into town for her graduation. Abby, Neela and Lester graduate from medical school. Steve continues his efforts to get Sam back, visiting Luka at work and sending Sam a box of ice cream sundaes. Pratt refuses to believe that a patient of his is in congestive heart failure. Weaver suggests to Abby that she will match at County, unless she fails her boards. Rachel Greene is in town "looking at colleges," although she has a favor to ask of Corday.

Good samaritan Luka stops to help a woman with a flat tire, and watches her get hit by a reckless driver. Abby discovers that she passed her boards. Pratt is concerned about Chen when she comes to work with a black eye. Neela withdraws from her residency program at the University of Michigan: Ann Arbor, knowing that she can't be licensed without an internship. When Steve announces his intentions to stay in Chicago, Sam decides to leave town. The custody battle for Henry Lopez goes before a judge. While Pratt drives Elgin Gibbs home en route to dinner with Chen, the three are shot at by an irritated driver.