When a drunk patient is admitted to the ER, his actions endanger everyone at County. Pratt is still pursuing his dreams of becoming Chief of the ER, and Gates is worried about his future with Sam. Luka unexpectedly runs into Dr. Moretti.

Neela's aortic rupture patient is in need of yet another follow up surgery, and they are starting to take a toll on her. Morris talks with a shrink about his experience with the jewelery thieves. Abby needs to make a tough decision when a new opportunity is presented to her.

There's been a jewelry robbery in town, and Neela finds out that one of her patients might have had something to do with it. Pratt makes an effort to save his relationship with Bettina, and Abby worries about her marriage, sensing that Luka may have given up on it.

Kovac is in Chicago looking for hospice-care work. Neela's aorta surgery has caused an infection in her patient. Abby treats a teen who has been beaten up. Brenner continues to cause problems for the ER staff.

Neela, worn out after her final ortho shift, has to stay on duty when a scheduling mistake has her surgical trauma shift starting immediately. Sarah's grandparents are in town, and Hank is taken to the ER after collapsing during an outing. Gates tries to convey to Sam that his feelings for her are serious. Pratt and Sam treat a father who shows signs of having been physically abused by his son.

Pratt and a new doctor on the scene disagree on how to teach the interns. Gates treats a young woman with serious heart problems. Abby heads to Croatia, and ponders whether and how to tell Luka about her unfortunate choices. Sam discovers that Julia has moved on in more ways than one.

The ortho docs face off in a hockey game against the trauma docs, who have enlisted the aid of a ringer. Bettina wants to take her relationship with Pratt to the next level, but Pratt is hesitant. Gates and Sam disagree over how to treat a young patient. Julia's attempt to provide spiritual guidance to a prison doctor and help ease his conscience leaves her devastated.

Harold seems to have won over Neela's cousin. After hammering out the details with County's administration, Abby is cleared to return to work. A fire sends two foster kids and a firefighter to the ER. After treating one of the children, Abby suspects the little girl is suffering from a degenerative eye disorder. At the end of the day Abby decides to come clean with her colleagues about where she's been since Luka's father passed away.

After being passed over for ER Chief, Pratt makes plans to leave County and go to a hospital that will appreciate his talents and take him more seriously. A woman comes in with a severely injured ankle and a terrible secret, and Sam finds herself closely identifying with her. A former ER staff member rushes in to be with her son, who was brought in after a fall during gym class. Neela's cousin is in town, and Morris and Harold vie for her attention.

ER's 300th episode finds Luka and Abby hurriedly preparing to travel to Croatia after Luka's father passes away. Luka confronts Abby about the estrangement between them, and she eventually reaches out and asks for his help. A man rushes to the ER when his wife is seriously injured in a car accident, and finds someone very unexpected there with her. Julia holds a service to bless the ER and remember the patients that have been lost through the years.

The staff must deal with changes in the ER when Anspaugh makes an unpopular decision. Abby has old demons to fight, distancing her from friends and family. A patient's racist son makes a deadly mistake. Two doctors secretly become involved. Gates has some adjusting to do when Sarah returns to Chicago. Luka gets bad news from Croatia.

Abby's attitude begins to affect and alienate the staff. Luka returns from Croatia a day early. Neela's intern makes her proud when he deftly handles a critical situation. Tony and Julia's tryst is interrupted when Sarah needs rescuing.

A power outage strikes on an unusually hot day, bringing nursing home patients suffering from hyperthermia to the ER. Abby and Pratt disagree about diagnostic tests for a fussy baby. Morris and Pratt get their board results, and the ER staff celebrates at Ike's. Moretti and Abby's interactions intensify, and Abby finds herself at a dark and frightening crossroads.

Abby and Moretti clash over how best to evaluate the interns. Morris and Pratt take over the lounge to do some last minute studying for the Boards. Neela asks them to assist her intern with a personal matter. Joshua comes back to the hospital in respiratory failure and Gates treats him. Sam is puzzled by a teen patient's changing symptoms and abrasive behavior. Moretti's son shows up for an unexpected visit. A new attending arrives at the ER. Abby continues to wrestle with her alcohol problem and it's consequences.

Morris is missing Hope and having trouble with thoughts of straying. Neela gets an intern and must find the best way to mentor him. Gates is given another chance in the ER, and quickly clashes with Kayson on a diagnosis. Abby hears from Luka and learns that his absence will be extended another week. Coburn tries to help a resistant Abby.

A patient's appreciative mother offers the ER staff free makeovers, with some interesting results. A new desk clerk is introduced. It's a bad day for Abby, missing the still-absent Luka and unable to reach him by phone when Joe is brought to the ER after a fall. Elsewhere, the day puts family first as Morris cares for a frightened child and Pratt defends his brother. Neela returns to work, and Moretti returns from a conference.

Two wounded police auxiliary officers present challenges in the ER. A recovering Neela settles in at Abby's and offers to help with Joe. Pratt and Bettina consider their relationship. Gates tries to get a patient on a drug trial. Morris receives a phone call that leaves him preoccupied and shaken.

Moretti makes his presence felt as he implements changes in the way patients are seen. Abby and Dubenko differ on how to proceed when a patient comes in with a bullet in him needed as evidence. Pratt complains to Anspaugh about the new Chief. Neela is released from the hospital. Abby has passport woes, and also struggles with the changes being made by Moretti. Hope prepares for a new mission, one Morris is not too happy about. Gates helps, and is helped by, a child prodigy in the ICU. Sam, Chuny and Dawn go "speed dating."

Moretti's first day is challenging as the ER is inundated with casualties from the peace rally - including Neela. Dubenko faces a crisis in surgery. Abby, Pratt and Morris conspire about how to deal with a shared problem. A new intern arrives unexpectedly, and Gates is reassigned.