There's tension between Meredith and Addison. We find out why Derek and Addison aren't together. Bailey puts George on an impossible case. The rest of the interns operate on a family that was just in a car accident.

Cristina and Meredith try to figure out if Cristina should tell Burke that she's pregnant right before Cristina faces an emergency. Alex and Izzie get closer while Meredith's mother gets too close.

Cristina goes right back to working right after being discharged. Izzie isn't talking to Alex. Derek and Meredith treat a patient. George and Alex get stuck in an elevator.

All the nurses at Seattle Grace Hospital go on strike, leaving the doctors to fend for themselves. George refuses to cross the picket line and finally joins the nurses on their strike while Alex, Cristina, Izzie and Meredith meets Bailey's stand-in: Dr. Cindy Aaron, the complete opposite of Miranda Bailey. Izzie agrees to work on a difficult pregnancy case with Addison, while Cristina and Alex are in the pit attending to a young married woman with a real bad case of rash. Meredith happens on an old woman that she needs to intubate and then learns she was DNR.

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.

George and Derek work together, Denny still doesn't have a heart, and Meredith gets paired with Callie. A 12-week pregnant girl dies in the O.R. and her parents try to keep the baby alive. This causes tension between Derek and Addison. One doctor's overreaction unites two bickering patients. Finn is still treating Doc for medical problems.

Denny needs a heart or else. Izzie and George go crazy trying to get him one. Derek has the life of a friend in his hands as he operates on Burke who has been shot. Burke cannot operate on Denny after Denny's heart stopped and now Denny is in a dire condition. Cristina gets put in charge of the ER and she is worried about Burke.

Meredith must make an important choice about which path she wants to follow to determine the future of her love life. While she considers her future she and others have flashbacks of their pasts. Cristina spends time with Burke while he recovers. Webber and Adele have a conversation after he spends the night at the hospital. Everyone attempts to deal with Izzie's choice to quit. George seeks relationship advice from Derek.

Cristina gets to scrub in on a rare surgery which incites envy. Alex questions his future in plastics. George and Addison work together with Noelle. Noelle is pregnant with twins but one of them is growing slower than the other. Her fiancé has problems of his own. Derek's sister visits. A patient's wife makes things hard on the staff since she has made her husband's health her job. A woman's sexual exploits are questioned.

A face familiar to one of the year-twos shows up and dredges up issues that were never really dealt with. A teen soccer player needs counseling because of his father and surgery for an injury he sustained while playing the game. Another patient's, named Courtney, finds out that her cancer is back.