Meredith takes a pregnancy test and it's positive! She tells Cristina immediately, but decides to tell Derek later that night.
Gary Clark, the man who blames Derek for his wife's death, arrives at the hospital. He asks where he can find the chief of surgery, but no one has time to help him. He corners Reed in a supply cabinet and when she brushes him off, we gasp as he shoots her in the head! And then he shoots Alex, who's just walking by. Alex manages to crawl to the elevator, even though he's bleeding severely.
Cristina asks Owen, "Do you love Teddy or do you love me?" and when he can't answer, she breaks up with him. Cristina's mind is still on Owen when Clark asks her where he can find Dr. Shepherd. She gives him directions to Derek's office. We hold our breath until the elevator opens, and then he tells her, "Have a nice day."
April finds Reed's body. She's hysterical as she runs into Derek's office covered in blood and tells him that someone shot Reed. Derek orders a lockdown of the entire hospital and leaves April in his office while he goes to warn Teddy and Owen, who are in surgery. He tells Jackson that there's a shooter in the hospital, but not to say a word to anyone else until the patient is stable.
Callie is barely speaking to Arizona after their breakup, but now they're sealed together in the pediatrics ward.
Although Meredith and Cristina know there's a lockdown, they have no idea why. Meredith decides she can't wait any longer to tell Derek about the baby and Cristina goes with her. Derek sees them and ushers them both into a closet, telling them not to move.
Clark loses his patience and starts shooting. Mark grabs Lexie and pulls her to safety. He pushes the button for the elevator, but when it opens, there's Alex, bleeding out! They carry him into the nearest conference room and examine him on the table. The bullet's still inside, so Mark has to get it out.
Miranda is with a patient when Charles comes in and tells them the shooter's on this floor! Miranda orders the patient to play dead and tells Charles to hide while she dives under the bed. Clark finds Charles in the bathroom. He asks if he's a surgeon and when he says he is, Clark shoots him! He then drags Miranda out from under the bed and asks her the same question, but she insists she's just a nurse. Clark apologizes and leaves. Miranda tells Charles, "We're all going to be fine," but he's sure he's dying. She swears she'd tell him if he were.
Despite there being a shooter on the loose, Owen takes his patient to the ICU and Teddy insists on going with him.
Derek sees Clark and warns him to get to safety. "I know it's not safe. That's the point. You don't recognize me do you?" Derek finally realizes who Clark is, and that he's here to kill him.
Meredith and Cristina leave the closet just in time to see Clark pointing the gun at Derek.
Derek is trying to talk Clark out of shooting him, saying, "I know your loss. Two guys killed my father for his watch, right in front of me. I didn't become a doctor because I wanted to be God. I'm a doctor because I wanted to save lives. I make mistakes. I'm flawed. We all are. Today, I think, for you, is just a mistake. You're a good man. I can see that in your eyes. Can you see it in mine?"
Clark lowers the gun, but just then April bursts out of Derek's office. As Derek turns to warn her, Clark shoots him in the chest!