Moretti reassigns Gates back to the ER and treats a couple involved in a car accident. Morris prepares to take his written board exams. Neela is assigned a young genius intern. Kovac plans to extend his stay in Croatia while Abby struggles with single parenthood.

Morris and Pratt cram for their medical boards when Morretti brings in a new attending physician to help with the load. Sam and Abby work on an attention-hungry patient who comes into the ER with elusive symptoms.

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.

Everyone at the ER is happily surprised to see Luka back from Croatia. Great news awaits Neela. Sarah once again needs rescuing by Tony, and patients are causing problems for Morris.

It's time for changes at the ER when Anspaugh makes an unpopular decision. Abby has old demons to fight with, which distance her from friends and family. Even Luka seems to be at a loss to help Abby. Love is in the air when a secret romance blooms between two doctors.

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.

After being passed over for Chief of the ER, Pratt is looking for a hospital that will appreciate his talents. A former employee of the ER brings her sick son in for tests. Neela's cousin is in town for a visit, and Morris and Harold compete for her affections.

Abby is back from rehab and finds herself the object of scrutiny. She confesses to Pratt that Luka might stay in Croatia longer than planned. Harold seems to have won over Neela's cousin. Multiple victims from a fire fill the ER.

Neela's short visit in Ortho turns into a game of hockey. Bettina wants to take her relationship with Pratt to the next level, but Pratt is not keen on the idea. Gates and Sam disagree over a patient. Julia provides some spiritual guidance in order to ease a prison doctor's bad conscience.

Neela, worn out after being on shift for over 24 hours, has to make a quick decision regarding an accident victim's torn aorta. Sarah's grandparents are in town, and Hank is rushed to the ER after collapsing during an outing. Gates tries to convey to Sam that his feelings for her are serious.