Carter begins the first day of his internship with the difficult task of filling in for Benton as the ER surgery consult. Jeanie makes difficult decision regarding her HIV-positive status; Peter's test comes back negative. Weaver implements an incoherent system for tracking patients. The ER staff attends a Fourth of July softball game, where Carol runs into Shep and his new girlfriend. Peter runs into an old friend, Carla Reese, at Jackie and Walt's barbecue.

Jeanie begins to realize the burden of living an HIV-positive life when bills start mounting up and Benton throws her out of a trauma. The staff nervously awaits news on the hospital's future when it's rumored to be closing. As it turns out, South Side is closed instead and County takes on a new chief of staff, Dr. Anspaugh. Mark and Susan endure disastrous blind dates. Benton sends Carter to the ER for the day after he arrives late.

Mark is befuddled by an invitation from Susan to go on a joint vacation to Hawaii. The ER is filled with more refugees from South Side, Drs. Maggie Doyle and Abby Keaton among them. Peter decides to join Keaton's pedes team. Carter gets on Anspaugh's bad side when he exaggerates a patient's condition to book an operating room. Jeanie tries to keep her HIV status a secret in the face of Weaver's queries. Jerry tries to trap a loose kangaroo. Mark spends his entire day working on one patient, against Anspaugh's decree that ER attendings and residents should handle 2.5 patients per hour. Carter's apartment building burns down.

Dr. Ross brings a one-night-stand into the hospital when she has an epileptic seizure, earning him the stares of his colleagues when he can't remember her name. Carter screws up Benton's lecture when he realizes his presentation materials were in his apartment fire. Keaton takes a shine to Carter, meanwhile, when he persuades a teenage girl to undergo surgery. Carter also finds a place to live, with Dennis Gant. Jeanie begins experiencing side effects from her medication and is asked out by a handsome construction worker. Carol ponders enrolling in med school.

Doug and Carol man the Healthmobile, a roving medical van that caters to the homeless. Benton tries to be more warm and friendly towards children, even taking a group of hospitalized youngsters trick-or-treating. Gant demands more respect from Benton. Mark anxiously waits for Susan to return from vacation.

Mark and Susan take to the air for helicopter flight rotation, which brings to the ER an entire family injured in an accident. Benton's overconfidence in Keaton's absence results in a dire mistake. Carol deals with a "floater" nurse who can't cope with the speed of the ER. Jeanie and Maggie are given the task of keeping a dead man on ice in anticipation of cryogenic storage.

Benton fights to save the life of a gangbanger already declared dead by Doug. Nurse Rhonda Sterling floats down to the ER again and makes another critical mistake. Carter fights Edson for the rights of an incompetent patient and bonds with Anspaugh in the meantime. E. Ray worries that an MRI test gave him negative karma. Carter and Keaton get closer. Mark fears Susan is dating Morganstern; she tells him that she's moving to Phoenix.

Susan's last day in the ER finds Mark unsure of himself. Despite Doug's urgings, Mark still finds himself afraid to admit his feelings for her. He chases her all over the city until he finds her at the train station. He declares his love for her, but she decides to leave for Phoenix anyway. Grabarsky and Lydia finally get married. Carol agrees to become "management" in exchange for an end to the policy of floating ER nurses elsewhere. Al Boulet delivers divorce papers to Jeanie. Doyle calls the cops on a pregnant woman who is piss drunk, trying to kill her unborn baby. On Healthmobile duty again, Doug meets a young girl named Charlie.

Mark, still reeling from Susan's departure, crosses ethical lines to obtain Jeanie's confidential files after learning Al is HIV-positive. Benton rides Gant hard. Carter and Keaton find it harder to maintain their secret relationship.

Rumors sweep the ER when Kerry and Mark cloister themselves to discuss policy regarding HIV-positive employees, prompting Jeanie to publicize her illness. Charlie brings the child she has been caring for into the ER. Doug treats the cihld and feels obligated to report the child's mother for neglect. Doyle treats an abused woman, and conspires to help her escape her husband. Carter blows off an evening with Gant to spend the night with Keaton. Mark inherits a mangy dog from a patient and gives it to Rachel for Christmas. Carol and her mother host a Russian holiday celebration, to which Doug takes Charlie.

A long, cold night finds ER staffers with hours of time to kill. Kerry recruits staffers to participate in a study on sleep deprivation combined with physical activity. Gant appeals to Anspaugh after a public scolding from Benton. Carter ditches Dennis again to escape his whining, only to discover deadly results. Peter is denied a recommendation from Keaton one last time before she leaves for Pakistan. Mark and Chuny discover that they have a lot in common. Carol is at a crossroads when management demands she cut two RNs. Charlie comes into the ER severely beaten.

Carter blames Benton for Gant's death. An overmatched Carol makes a fatal mistake when the nursing staff is on a sick out. Jeanie dates infectious disease specialist Greg Fischer. Mark and Chuny continue their relationship. Dr. Ross tries to help a non-cooperative Charlie.

Carol takes an interest in an ailing police sergeant who's hiding a degenerative ailment. She also meets with a newspaper reporter to air the complete story on her fatal mistake and is subsequently suspended. Peter runs into a pregnant Carla, upsetting the rest of his day. Carter, already fed up with Benton's lackluster attitude, accepts a transfer to Hicks' team after Peter fails to show up at a lecture the two were to give. Jeanie and Greg Fischer disagree on whether to inform a patient's spouse that he's suffering from neurosyphilis. Weaver and Mark each take a trio of prospective students through a day in the ER, with widely differing results. Chuny and Mark break up; however, he has another date by the end of the day.

A gleeful Carter removes Benton's appendix. Dr. Ross treats a 17-year-old cystic fibrosis patient who wants to die, but isn't old enough to sign a DNR. Mark juggles dates with three women, including psychiatrist Nina Pomerantz. Jeanie and Greg team up to find the culprit behind a rash of staph infections. Haleh has a tough time filling Carol's shoes.

Carol is held hostage in a bungled convenience store robbery. Her medical skills are put to their toughest test as she works feverishly to save the gravely wounded store owner.

Dr. Greene tries to get a congestive heart failure patient with Down's Syndrome onto a heart transplant list. Carter disagrees with a diagnosis by Anspaugh. Carol agonizes over whether to take the med school admittance exam. Jad Houston returns to the ER, old enough to decline emergency care. Greg and Jeanie agree to continue seeing each other.

Mark examines his prejudices when he's accused of neglecting a black gunshot victim, Kenny Law, in favor of a white victim. Jeanie discovers Peter's impending parenthood when Carla comes into the ER. Carter spends the entire day treating a hopeless case. Kerry tries to persuade a disabled junkie to enter a rehab program. Carol treats a college student who was drugged with roofies. After Jenn's mother has a stroke, Rachel comes to stay with Mark for a while.

Carter goes behind Anspaugh's back to get a gambler admitted for surgery, earning Anspaugh and Hicks' wrath and jeopardizing his residency. Carol clashes with Doyle, angered that a 25-year-old intern is able to order her around. Peter illegally obtains Carla's ultrasound to look for abnormalities. Doug treats an ill teenager who doesn't want to miss her prom. Greene deals with a patient who feigns symptoms because she likes having surgeries. Jeanie faces her feelings towards Al when she cares for a remorseful AIDS patient who infected her child. Jerry hits on a way to make money from a sperm bank: pass himself off as "Dr. Markovic."

Carol finds herself at a career crossroads when her MCAT results come back. She's torn between nursing and becoming a doctor, with Kerry grooming her for medicine, while Carol tries to care for a mother with a dying child. Carter debates whether to report Edson for falsifying a patient's chart. Benton takes a personal day to care for Carla. Dr. Greene treats a couple concocting odd situations to see how quickly the staff finds a solution. Jerry, Wendy and Jeanie search for a genetically engineered mouse.

The staff springs into action when Dr. Greene is violently beaten by an unknown assailant. Dr. Ross treats the son of a wealthy man, leading Carol to suspect Doug of giving the child special treatment. Greg demands that Jeanie choose between him and Al. Speculation runs rampant when a torrid novella, author unknown, is found at the admit desk. A new pediatrician arrives from Philadelphia. Olbes shows up out of uniform for a change, bringing in her grandfather, who has suffered a stroke. Carla is admitted to the hospital after she goes into premature labor; the contractions are stopped for now.

Mark returns to work, scarred both physically and emotionally. Carla goes in labor again, four weeks prematurely, and delivers a baby boy. Doug thwarts Carol's efforts to quietly celebrate her birthday. Carter begins to doubt that his future lies in surgery.

Mark, still recovering from the attack, vents his emotions in a violent fit in the break room and buys a gun for protection. Carter decides his future lies in emergency medicine, not surgery, and tries to convince an irate Anspaugh likewise. Drs. Ross and Del Amico work together when Charlie is brought back into the ER. Peter's child strengthens. Doug and Carol share a kiss. Jeanie and Al are back together.