Train Wreck victims come to Seattle Grace and all staff gets called in. Everyone deals with really weird cases. Except Addison and Izzie. They work with a woman who is pregnant.

Meredith tries to avoid the Shepherds. Cristina and Burke go out on a date and end up getting caught up in their jobs. Derek asks Meredith not to ignore him until Addison comes walking around the corner.

Meredith and George are experiencing morning-after tension at home and at work and it has the rest of the doctors wondering what's up. Derek and Addison are experiencing their own version of morning-after tension as a result of Mark's visit to Seattle. Bailey, returning the favor that was done for her during the Code Black situation, must treat Addison for a specific feminine problem. Izzie and Alex are making headway in their relationship until Denny Duquette comes back. Soon Alex is not-so-oddly jealous of the attention Izzie is showing to Denny. Someone finds comfort in confiding in a patient who is going through an ordeal of his own. Burke deals with a patient and gets to observe the patient's unstable relationship with his fiancée.

The doctors are up to their necks in their usual levels of drama. Meredith is dealing with the backlash of sleeping with George, George and Burke are bonding in ways that bother and creep out Cristina, Cristina gets stuck on diaper duty, Bailey gets involved in a case even though she's still supposed to be on maternity leave, Addison and Derek have a talk, and Izzie is still flaking on Alex for Denny Duquette, the heart patient who didn't get a heart. In the patients-of-the-week department we've got a very pregnant woman, a hockey player who desires medically unnessasary amputation, and a woman with a fork in her neck who happens to be dying and has a struggling marriage.

The staff attends to an intern from another hospital who has short-term memory loss. This intern, named Marshall, also has caused a car accident that involved a married couple, their unborn baby and the woman's parents who the staff are also treating. The pregnant woman is named Melanie and she will die but her baby may survive. One of the female interns meets up with a woman from her past who is planning on having a family of her own. Cristina is in a good mood although Alex, Izzie, George, and Meredith are having problems but in the meantime Burke hasn't forgiven Cristina for her late night faux pas. Meredith and Izzie are making fun of Callie's household habits and George and Callie are having a fight because of George's roommates. Addison is continuing to own Alex and he's upset about it. Denny has an outburst at Izzie and Derek has an outburst at Meredith.

George's father has a serious heart problem that requires surgery and George chooses Burke to do the procedure. Burke doubts himself because he knows the pressure this kind of surgery will put on him, but Cristina believes the team can handle it. Derek works with Meredith on being a couple again and tries work with Addison on more civil terms. A patient has had mutiple plastic surgeries so that his girlfriend will like him more. A mom wants her daughter's babysitter fired, after she is brought in with multiple injuries, but the girl likes the babysitter more than her mother.

A patient is in the hospital because they fell asleep in a hazardous situation. A patient's home laboratory blew up in an apartment building and the victims flood the hospital. Alex investigates the cause of the incident by talking to a really old guy who lived in the building. Jane Burke shows up to deal with what her son left behind. Lexie tries to talk to Meredith. Callie is overwhelmed by her new position while Bailey is searching for things to do.

The doctors meet the world's oldest intern. Marcy brings in her son Henry because she finds his change in behavior suspicious. A patient named Connie may lose her vocal chords unless Mark and Richard can perform a radical new surgery. George impresses the other interns at work but at home he is trying to deal with his guilt about having sex with Izzie.

It's Halloween at Seattle Grace! There may be jokes, pranks, and/or scare tactics. Some of the women form a club based on an experience they all share. A man volunteers for what most of the doctors believe is an unnecessary surgery. One of the patients is in the hospital because they tried to carve a pumpkin in an unconventional manner.

There has been a bus crash and the patients are in the ER. Meredith and Derek's relationship gets more complicated when he dates a fellow doctor. Meredith's dad goes to visit her at the hospital.

An ambulance crash endangers the lives of the paramedics involved, as Meredith and the Chief work on-site to save them, Bailey treats a patient who refuses her help, and Ava returns for Alex and meets his other girlfriend in the process.

Meredith works furiously to save a paramedic trapped in an ambulance; Lexie holds the life of a patient in her hands; Derek must rely on a nurse to save a patient; Bailey struggles to balance her personal life with her work.

The hospital staff is left to deal with the aftermath of George's passing. Hitting the staff hard, they all find unique ways to get through the various stages of grief. George's mom returns, faced with the difficult task of deciding what to do with his organs.

The residents of Seattle Grace and Mercy West vie for surgeries and their careers as day one of the hospital merger begins. Meanwhile, Arizona stands by an enraged Callie when her father, Mr. Torres, employs a priest to help convince her to date men, and Izzie experiences the brunt of her new surgical competition.

A gunman opens fire at a local college, sending mass casualties to the hospital; Arizona's return is not warmly received.

After a roller coaster car falls off the track at the county fair, the doctors at Grey Sloan tend to patients who spark memories about ghosts from their past. The series 300th episode!

Meredith faces the medical board as her future as a doctor remains uncertain, and she’s forced to reckon with her past in some challenging ways. Meanwhile, the interns are put to the test as they are each vying to be the most successful on their respective cases in the absence of some of the attendings.