<P> The hospital is hit with an unprecedented crisis: a shooter is in the hospital, and there is a lockdown . Meanwhile, Dr. Meredith Grey (Ellen Pompeo) discovers that she is pregnant, and Dr. Owen Hunt (Kevin McKidd) must choose between Teddy and Cristina (Sandra Oh). The shooter kills Reed Adamson and wounds Dr. Alex Karev (Justin Chambers). Lexie (Chyler Leigh) and Mark (Eric Dane) find the latter and try to save him . Soon, it is revealed that the shooter, Gary Clark (whose wife was a patient at the hospital and later died), is looking for Dr. Derek Shepherd (Patrick Dempsey). At the end, he shoots Derek, which is witnessed by Meredith, Cristina, and April . With the crisis unfolding, each character is put through extreme trials and tribulations . Cristina is put under pressure to save Derek, who is seriously injured . </P> <P> Meanwhile, Dr. Miranda Bailey (Chandra Wilson) and her patient Mary drag intern Charles Percy, who has been shot, through the hallway but when they get to the elevators, they are off . Bailey freaks out for a moment, then gathers herself and sits with Charles . When he asks if he was dying, she said yes . She tells him she and Mary are going to stay with him and that he isn't alone, with Charles dying soon thereafter . The shooter walks into the OR where Cristina and Jackson are trying to save Derek . Hunt shows up and Meredith tells him what was going on . He looks through the window and says she is doing fine but he'd go in to see what he could do to help . When Hunt goes inside, the audience is shown that Mr. Clark is in the room with a gun to Cristina's head, shouting at her to stop fixing Derek . Cristina and Avery refuse to stop working . </P> <P> Mr. Clark tells Hunt that he'd shoot him first and then Cristina but he really only wanted to kill Derek . He believes this is retribution for his wife's death, which he believes is on Derek's hands . Meredith then comes into the room and tells Mr. Clark to shoot her . She explains that she is Lexie's sister, she is the closest thing Webber has to a daughter, and she is Derek's wife . He turns the gun toward Meredith, but Cristina says Meredith is pregnant . Hunt makes a move toward Mr. Clark and is shot, knocking him unconscious . Cristina and Avery then raise their hands and Avery tells Mr. Clark that Derek will die and he can watch it happen on the monitor . Derek flatlines and Meredith cries almost hysterically . Mr. Clark walks out . </P> <P> Clark later comes across Lexie, who is bringing supplies to help Alex, and when he attempts to shoot her, the recently - arrived SWAT team wounds Clark . This gives time for Lexie to escape . Soon after, Webber walks through the halls of the hospital and finds several bodies . In a room, he finds Mr. Clark, who tells him he'd bought the gun at a superstore and bought extra ammo because it was on sale . Mr. Clark tells Webber he has one bullet left . He reveals that he planned on shooting Webber, then himself . Eventually, after Webber convinces Clark that his wife wouldn't want the shooting and would rather be reunited with him, Clark commits suicide as the SWAT team is directly out the door . </P>

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