At home, Derek tells Meredith that he and Christina went fishing and she cried - that's all that happened. Meredith sulks that it's not okay for him to steal her best friend.
Christina and Owen are in bed at home. She says she wants to explore the city today since she's never had time before. Christina leaves the room and Owen gets a mysterious call from Teddy.
Mark and Lexie wake up together at her place and kiss before getting up to dress for work.
Callie leaves her apartment and finds Arizona sitting outside the door. She asks if she sat there all night. Arizona admits that she left and came back. Callie advises her to go back to Africa.
Owen arrives at city hall to meet Teddy and Henry the dying patient she intends to marry. Owen has some doubts about the plan, but follows them for the ceremony.
At the hospital, Arizona explains to Dr. Webber that she's made arrangements in Africa, but he seems to imply that she can't have her job back. Teddy and Owen arrive arguing about her wedding. They stop short in the foyer where everyone has gathered to watch a report on a mass shooting at a college.
Christina is walking in the city when she hears numerous sirens. She follows them to the site of the mass shooting. As she watches the traumas coming out, she begins to shout suggestions.
Back at the hospital, the stunned staff prepare for the influx of shooting victims. A cop tells Webber the shooter went down and they need to get his injured officer talking to identify him. Owen greets an ambulance. The doors open to reveal Christina working on a victim. She says, "I had to crack his chest!" Teddy takes the man into surgery and Christina follows, but Teddy calls Jackson in for back up - she's worried Christina won't be able to handle it.
As Dr. Stark operates, Arizona hollers instructions from upstairs. Stark becomes angry and tells Karev to get out. Arizona orders Karev to block Stark from removing the patient's leg. She runs to Webber, who is trying to get the injured officer to come around and give details about the shooter. Webber gives her privileges. Arizona heads back to the operating theater where Callie has arrived and is looking at the leg. Arizona bursts in and says she will be operating, not Stark. Dr. Stark rushes to find Webber, who furiously orders him to be a doctor and go save a life right now! Webber arrives in the OR where Teddy and Christina are operating. He tells them they are working on the shooter. Jackson begins ranting about them wasting time on a guy who shot up innocent people. He walks out, as does a nurse. Christina stays with Teddy. After, Teddy thanks Christina, who says it was an easy decision.
Meredith is assisting Derek while he works on a man's brain. She wants update his distressed wife. Derek is irritable with her, but she goes. When Meredith returns, Derek says he has to remove both sides of the man's skull. She wants to tell the wife. Derek balks, but she hollers that she was the wife in the waiting room once - or is he so busy supporting Christina that he forgot that? Meredith goes to fill in the wife, and then takes her to the window. The college alma mater are assembled outside singing. Meredith returns to the OR for Derek. Together, they go to tell the wife her husband is in a medically induced coma, she can see him.
Webber returns to the police officer to find he's bleeding in the belly. Owen tells him to move to a trauma room. Jackson appears and rants to Owen about the shooter taking up an OR. Owen goes to look in on Christina. The healthy police officer goes to the waiting room and breaks the news to the young man's mother that he is shot, and was the shooter. Jackson watches as she sobs. Webber is able to save the cop, and tells his relieved superior officer.
In their OR, Callie and Arizona work to save the man's leg. Arizona starts personal talk, but Callie makes her stop. After, Arizona tells Callie she is deeply sorry for hurting her - she loves her and will spend the rest of her life telling her that. Callie responds that she left without a backward glance, and her feelings haven't changed.
Mark is working on a woman and allows Lexie to save her life. Down the way, Karev's patient, a boy, crashes. Bailey and Jackson help. Bailey hollers when he flatlines and suddenly he is okay. Jackson seeks out the mother of the shooter, who is alone. She says she had no idea about her son, but he's her baby. Jackson gently updates her.
Outside, Derek tells Meredith that she amazes him. They hug, and he tells her that the fishing worked - Christina is in the OR.
The entire staff assemble in the viewing area to watch Christina and Teddy work on the shooter. They save his life. Webber remarks that of 26 casualties, no one died today. Some of the staff cry. Suddenly, they begin to laugh. Dr. Stark walks out, saying, "I hate this place." Webber tells Arizona he'll hire her back - but she has to work under Stark. As she grimaces, everyone bursts into fresh laughter.
Lexie finds Mark, who is still working on a patient. She tells him she loves him and will wait. Meredith finds Christina in the corridor and she asks her to get a drink. Meredith agrees, but doesn't want a real drink because she's trying to get pregnant.
Teddy meets Henry at the bar. She wants to talk about the insurance, but he wants to share a drink first. She tells him about her day. He says she saved his life too, and that deserves a toast.
Spoilers for Next Week on Grey's Anatomy:
Callie drops a bombshell
Callie orders Arizona out of her apartment.
Competition ensues among the residents.
Meredith and Derek focus on conceiving.
Wow! What an excellent and powerful episode tonight. Not only was the drama riveting, but there was the release of tension at the end of the episode we have come to appreciate and expect on Grey's Anatomy. Mark and Lexie fans will be happy that they are 'on' again, and even I was moved when she told him she loved him. Callie's proving to be a tough nut to crack for Arizona - I hope this doesn't drag on. I loved the end with Derek and Meredith and then Meredith and Christina. A perfect episode - with even a laugh at the end courtesy of crusty Dr. Stark!