Part one of the two-hour season finale. Gary Clark enters Seattle Grace with guns to seek revenge for his wife's death. He kills Reed Adamson with a single bullet to the head and shoots Alex and then leaves to find Derek, whom he holds responsible. The hospital goes on lockdown and a pregnant Meredith and Cristina hide in a supply room. Lexie and Mark find Alex heavily bleeding on the floor while Owen, Teddy and Jackson are in the OR operating on a patient. Bailey and Dr. Charles Percy are with a patient named Mary, when Clark comes to their room. Bailey and Percy hide while Mary plays dead, but Clark finds Percy and shoots him, but spares Bailey's life when she says she's a nurse and he leaves. Callie and Arizona are stuck on the Pediatric Wing and have to take care of all the children as well as dealing with their own recent break up. Cristina catches a glimpse of Clark and remembers who he is and who he's after. Meredith and Cristina set out to find Derek, but Clark finds him first. Although Derek is first able to calm him down, Clark changes his mind and shoots Derek and then leaves.