Pilot: A day filled with traumas packs the ER. Under pressure from his wife, Chief Resident Dr. Mark Greene considers leaving the ER to join a private practice that would give him a much higher salary. When a building collapses, multiple casualties are brought in. New medical students start their ER rotation, among them John Carter, an inexperienced third-year student. As the day continues, Dr. Peter Benton operates on a patient with an aortic aneurysm, a procedure normally performed by senior staff. Head nurse Carol Hathaway is brought into the ER after a sleeping attempt.

Series Premiere: Carter's first full day finds him saving a patient from cardiac arrest. He later treats a flirtatious patient's rash and winds up going home with her. Meanwhile, the ER is inundated with a tour group and a wedding party suffering from food poisoning from the same Haufbrau. When Mark helps Jenn celebrate passing the bar, they get caught in a compromising situation in the ER bathroom. Doug visits a recovering Carol.

Carol returns to work despite some initial reservations and the staff greets her with a surprise welcome back party. Mark treats a domestic violence victim, while Susan goes on the defensive after a cardiac case goes awry. Benton is shadowed by a patient singing show tunes.

Carter gets emotionally involved with a high school hit and run patient who couldn't be saved and initially notifies the wrong parents. Benton disappoints his brother-in-law Walt when he is too busy to take care of his own mother. Doug attempts to reconcile with Carol, and also protects a pregnant teen's confidentiality. An older man complaining of chest pains arrives, while an obnoxious salesman insists on continuously using his cell phone, which trips a woman's internal defibrillator and causes an electric wheelchair to go crazy. A thankful patient gives Greene a dozen frozen T-bone steaks for saving his life exactly a year ago, proclaiming that every August 25 is Dr. Greene Day.

A business contractor with a bad heart gathers his family around him in the ER as he prepares for the end. Doug helps an asthmatic teenage girl obtain the needed medication when her mother is unable to afford it. Carter worries that he may have contracted an STD from Liz. Susan treats a teenager with alcohol poisoning and Jenn prepares to move to Milwaukee and start her new job.

The air conditioning quits on the hottest day in October which, coupled with closed ERs in two of Chicago's other hospitals, leads to an extremely hectic day. With his wife out of town, Mark is forced to bring his daughter Rachel to work with him. Doug treats a six-year-old suffering from a cocaine overdose. Ivan Gregor, a liquor store owner, shoots and seriously wounds a young teenager. Susan's immature sister Chloe drops by, wreaking havoc in her life. Doug picks up a pharmaceutical representative, much to Carol's displeasure.

Benton and Langworthy interview for the Starze Fellowship, after which Morgenstern praises Benton for saving a man with a throat wound. A drunk Chloe throws a tantrum in the hospital. Carter has a good day, winning a bottle of champagne and getting three leads on an apartment. After a spontaneous kiss with Doug, Carol has second thoughts about moving in with Tag.

Carol treats a rape victim, while Walt brings Benton's mother to the ER with a sprained ankle. Mark calls in sick to spend time with his wife, leaving Doug in charge. Div shows signs of snapping, overreacting at the slightest provocation. Benton learns he was not awarded the Starze Fellowship. The ER gets a new aide, "Bob."

Carter and Benton treat a suicidal transvestite. Doug feels like a kept man in his relationship with Linda Farrell. A patient entrusts Carol with the particulars in an accident. Div continues to battle his demons, ending up wandering down the middle of a crowded street.

A severe blizzard leaves the ER utterly devoid of patients, leaving the staff free to goof off. In the middle of their fun, a call comes from dispatch, informing them of a 40-car pile-up with dozens of casualties. The ER goes from being deserted to capacity as all surgeons are called in. Bob surprises everyone when she makes a quick, life-saving decision. A new doctor, Angela Hicks, arrives with perfect timing. Doug and Linda return from the Caribbean just as Carol makes a surprise announcement.

Benton makes a critical error when he publicizes the availability of a dying man's organs before obtaining the wife's permission to harvest them, and the wife then refuses to give it. Susan discovers Div has dropped out of sight, having both quit his job and moved out of his apartment. Carter makes a move on Susan, and Chloe announces her pregnancy.

Carter fumbles his first opportunity to observe a surgery. Susan's nemesis, Dr. Kayson, makes her New Year's Day decidedly unpleasant. Chloe announces she's moving to Texas with her boyfriend. Peter and his sister try to sort out care for their ailing mother.

Carter trains Benton's new med student, Deb Chen, while Carol has her hands full with an emotionally challenged patient who's obsessed with colors. A sociologist provokes people so he can measure their reaction times for a study. Kayson brings charges against Susan in the Vennerbeck passing, leading Morgenstern to question her assertiveness. Benton and Walt hunt for Peter's missing mother.

A case of apparent spousal abuse is not what it seems. Deb outshines Carter after he is up all night preparing for a presentation. Benton informs Jackie he's hired a helper for his mother. After Kayson's reprimand against Susan is denied, she has a chance to prove herself when he is brought in as a patient after suffering a heart attack. Doug meets Diane Leeds, who works in the hospital's Risk Management division, and her son, Jake.

Mark is invited to join the staff as an attending physician. Later, he treats a woman with breast cancer, who begs him tohug her. Deb wows Benton with her presentation, leaving Carter in the rear again. A poisonous snake gets loose in the ER. Benton offends Haleh, who spends her day making his job more difficult. The ER gets new crash carts, which are promptly stolen by cardiology.

Valentine's Day finds Carol and Doug caring for Tatiana, a six-year-old Russian girl with AIDS abandoned in the ER by her adoptive mother. Mark and Susan care for a dog hit by Officer Grabarsky. Three cheerleaders come to the ER after ingesting chocolates laced with LSD, which Deb later accidentally eats. Wendy's hand is broken in a trauma by a patient having a seizure.

Dr. Hicks berates Benton when he tries to switch shifts to attend his mother's birthday party. A teenage girl, a frequent patient of the ER, is brought in for a drug overdose. Carol wants to adopt Tatiana, but Tag isn't so keen on the idea. Doug slugs an abusive father in the lobby. Jenn accepts a job offer in Milwaukee despite Mark's offer from Morgenstern. Deb and Carter await the arrival of sub-internship applications.

Benton pulls a 48-hour shift and refuses to sleep, despite Hicks's orders, which later proves disastrous for his mother. A doctor spends the entire day studying the ER as a model for a new hospital, and offers Susan a job; he turns out to be a psych patient who wandered downstairs. Jenn tells Mark she wants a separation. Doug treats an abused young girl, and Carol receives bad news about her attempt to foster Tatiana.

An routine day filled with ordinary traumas becomes nightmarish when a distracted Mark critically misdiagnoses a pregnant woman's delivery complication. A teenager is brought in after being accidentally poisoned. Benton grapples with his own demons in the wake of his mother's collapse.

A new Chief shakes up the ER and is impressed with Susan. Mark remains deeply affected by the loss of a patient. Benton begins to realize his mother's future is not bright, and clashes with Tag regarding her hospital stay. Carter and Deb experience the strangeness brought on by the full moon coinciding with the naturally heavy Saturday night traffic.

Deb and Carter compete furiously for procedures after Benton announces their books are due a week earlier than previously announced. Deb commits a nearly deadly error on a patient while trying to keep up with Carter. Doug's relationship with Diane Leeds grows. Benton and Jackie decide to put their mother in a home. Mark is put on trial, so to speak, in the M&M conference for Jodi O'Brien. Chloe returns.

Susan has her hands full with a pregnant Chloe. Benton goes to dinner with Jeanie. Mark further alienates Swift when he decides to honor a family commitment rather than stay longer at the ER to help him. Carter makes a good impression on Dr. Swift. Doug turns down an offer to "celebrate" with Linda Farrell, instead getting more serious with Diane as he agrees to coach Jake's Little League team.

Mark and Jenn struggle to make their relationship work after he moves in with her in Milwaukee. Doug coaches Jake's Little League team. Chloe's irresponsibility wears Susan down. Carter decides to apply for both the ER sub-internship and the surgical sub-internship. Carol and Tag discuss their vows and other wedding details; he becomes upset when Carol invites Diane Leeds, knowing that she'll bring Doug. Benton exhibits a growing attraction to Jeanie. Susan discovers what happened to Div when she spies his picture in a cabbie's dating service photo book. The staff finds out that Carter comes from a wealthy family. Swift takes over for Mark during a trauma,who then bolts from the room.

Susan is forced into the role of parent when Chloe gives birth and both Chloe and her mother, Cookie, shirk responsibility. Doug is surprised and a bit frightened when Diane suggests they move in together. Unfortunately, Linda Farrell happens to be around to take advantage of his nervousness. Carter is offered the ER sub-internship but turns it down in anticipation of receiving a surgical position. Benton receives unpleasant news; his mother has died.

Season Finale. Carter faces his last day in the hospital, harshly grading Benton on an evaluation report. He later rescinds the report after finding he received the surgical sub-internship. Carol faces wedding day jitters, culminating in Tag deciding to call off the wedding. Chloe disappears again, abandoning little Susie with Susan. Dr. Swift changes his mind and decides to support Mark. Jake visits Doug.