Season Premiere: The 15th and final season of ER begins with the devastating repercussions of the ambulance explosion. As Morris and Neela do everything they can to save a wounded colleague, Abby treats a young girl injured during the blast.

Everyone in the ER is trying to cope with the loss of their colleague. The staff feels defensive toward the new Chief and her tough disposition. Gates and the new interns treat a bio terrorist, who has a lethal secret that endangers them all. Neela makes a great effort to get Dubenko back on the surgery staff, and gets help from an unexpected souce.

Abby's first day as an Attending is bitterweet. Neela is hurt when she finds out Abby's news from someone else. Abby and the new ER Chief argue over how to treat aballoonshot victim. Abby comes to Sam's aid when she faces a nurses' disciplinary board.

Dr. Banfield is caught off guard when she must defend herself during a robbery. Dubenko is reinstated due to Brenner's organized protests. A young gymnast comes into the ER with injuries consistent with a fall, but Banfield and Gates think a family member may actually be to blame. Sarah neglects to ask Gates' permission before getting her tongue pierced.

Sam's time is taken up by her studies for a pharmacology exam, leaving Gates feeling neglected. A school fight results in a stabbing, with Brenner treating the victim and trying to uncover the truth behind the story. Morris has a surprise for Neela. Gates gets locked in a stairwell with a quiet young boy on Halloween night.

Chaz, now a med student, begins his ER rotation. Morris takes him under his wing and shows him some risky procedures, raising Banfield's ire. Neela becomes upset when Banfield questions her authority and skills in front of her intern. Gates gets in hot water with Sam when he oversteps his bounds.

Episode summary coming soon...

Gates goes out looking for a homeless war hero and upsets Sam. Neela's med student gets the entire surgical department into a lawsuit. A couple comes into the ER after an attempt was made at burning down their home. Brenner gets into a fight with a patient, and later confesses a secret to Morris.

Neela must testify in court and defend her actions regarding a patient who died. Sarah and Alex defy Sam and sneak off to a party, with dire consequences. A snowstorm strands Banfield and Morris at a Nebraska medical conference.

A package for Morris from his deceased father is delivered to the ER. Sam has some anxiety over Alex being removed from the ventilator. A woman with a high risk pregnancy worries that she will be deported. An old friend returns to celebrate the holidays.

When a drug deal goes bad, two brothers sustain serious injuries and are brought to the ER. Tensions continue between Gates and Sam, coming to a head when Tony is the last to know Alex is about to be released from the hospital. Neela has to tell a young girl with sickle cell anemia that she must have an operation, and the case leads her to consider specializing in pediatric surgery. Banfield ponders her choices about expanding her family.

Neela has recurring dreams about trying to save a girl with sickle cell anemia. In them she also goes to an attending interview and sees an old colleague, has conversations with Ray, welcomes Brenner back from Australia, and assists Gates and Banfield in trying to save a "dream runner" who jumped out of a window during his sleep.

Morris faces a complicated case involving a young girl hit while riding her bicycle. Gates finds it very difficult to let go of his feelings for Sam. Neela becomes quite flustered when Brenner returns from Australia.

An old colleague visits the hospital to see a former mentor who is seriously ill. Sam's sister shows up unexpectedly. Gates and Morris tend to an elderly man who was assaulted on the street. A patient's diagnosis shakes Brenner up.

A car accident in the ambulance bay leaves a mother severely injured. Morris gets the run around from an insurance company. Neela and Brenner discuss her job offer.

Valentine's Day in the ER finds Neela spellbound by Brenner. Determined to have a baby, Banfield comes up with a plan but has difficulty getting her husband on board. Neela performs surgery on a young girl's mother. Dr. Carter surprises the staff with a visit to the ER. Gates treats a strange patient who may have ingested a toxin while hunting for mushrooms.

The Banfields explore their options with an adoption agency. Carter holds his own in spite of the procedural and technological advances made to emergency medicine since he's been gone. Brenner has concerns about Lucy's foster family. Morris treats a couple who takes his advice a little too far. Sam gets quite the shock when her mother comes into the ER. Neela makes a decision about her future. Banfield discovers Carter's secret.

A camera crew shooting footage for a documentary interviews members of the ER staff. Detective Claudia Diaz is brought in, critically injured withballoonshot wounds. Neela laments the lack of communication between her and Brenner. Sam struggles to reconcile with her mother. Carter's condition worsens quite suddenly.

A former mentor and colleague visits Carter in his time of need as he waits for a kidney transplant. Meanwhile, a woman is grief-stricken over the sudden passing of her grandson. Sam and Neela are stuck at the airport with transplant organs in hand. Banfield bonds with a baby abandoned at the ER.

Neela proves her worth as attending material when she clashes with Dubenko on how to proceed with two different surgical cases. Brenner gains a deep insight into his painful childhood.

Doctors and nurses from the ER help out at a camp for children who have had open heart surgeries. A woman returns to County for the baby she previously abandoned there.

Series Finale: Gates works on a young teen suffering from alcohol poisoning after playing a drinking game. Old friends come show their support when a new medical facility for the underprivileged is opened. Sam receives a special gift for her birthday.