So we pick up exactly where we left off, with Derek bleeding out on the walkway. April manages to blurt out enough personal details about herself that Gary doesn't shoot her and he has her run off. Cristina tries to get Mere to keep hiding but Mere throws her out of the way and goes to Derek, and fortunately Gary is gone by then. They decide to take Derek to an OR (and get April's help after Cristina finds her hiding) where Teddy will be so that she can operate on him. What they don't know is that Teddy and Owen left to try and get their patient to the ICU, but they realize that they can't make it there, and while pondering what to do Teddy tells him that he has to make a choice between the women he loves. He tries to pull a Kelly Taylor and choose no one, but once they are evacuated and he realizes Cristina is still inside, Teddy tells him it's okay to choose, and he gets past the cops and runs back in.
While Mark waits with Alex, Lexie is wheeling a cart of supplies back to the room when she comes face to face with Gary, and she was one of his intended targets. Luckily, he's a chatty gunman, and so there's enough time for a SWAT team member to get in place and shoot Gary in the shoulder so that Lexie can get away. Back in the conference room she tells Alex she loves him, which breaks Mark's heart, but as he gets worse and worse Alex hallucinates that she is Izzie and apologizes to her for everything he said and did, begging her to never leave him again, which breaks Lexie's heart. Eventually they are evacuated and Teddy takes Alex to a neighboring hospital, where it appears that she saves his life, and Mark dies a little more inside when he gets there with Lexie and she drops his hand to go run to Alex's bedside.
It's not all breaking hearts, though, because Callie and Arizona are still stuck in peds trying to help their terrified little patient with the burst appendix. Gary comes in and tells them he's been shot, so while Arizona hovers over her patient and tries to soothe her Callie talks to him calmly and gives him bandages and advice of what to do with them, which is enough to keep Gary from shooting them. After he leaves Arizona holds the girl while Callie talks soothingly, telling the girl about how she's got the best doctor in the world with the best smile that can make anyone feel special, and the two stop fighting and realize they still love each other. Once they all are evacuated, Callie tells Arizona that after nearly dying she realizes that she'd rather be with Arizona than have kids, but Arizona tells her they are going to have a huge family and they kiss and make up.
Bailey, meanwhile, is still trying to help Percy with Mandy's help, and they realize he's bleeding out and they have to get him to an OR. They manage to drag him to an elevator on a sheet, but the elevators have been turned off in order to try and trap Gary, and Bailey finally loses it completely for a few moments. She manages to regain her composure, though, so that she and Mandy can sit with Percy and hold him until he dies. With his last words, he makes them promise to tell Reed that he was crazy about her, not realizing he's going to see her sooner than they are.
When Cristina, Mere and April get Derek to the OR they find Teddy and Owen gone, but Avery is there and helps Cristina realize she's got to be the one to operate -- conveniently, the anesthesiologist and two surgical nurses are still there as well on account of the lockdown. Cristina does it but makes Meredith sit in the scrub room not looking, because she knows she won't be able to concentrate with Mere watching her, and has April stay there as well to keep Mere from peeking. Owen comes in and finds them there, and as he looks in through the window Mere explains the situation. She's a little surprised when he goes in without scrubbing in, but he promises he's going to assess the situation first -- what she doesn't know is that the situation is now Gary pointing a gun at Cristina's head and demanding that she stop working and let Derek die. She is totally freaked out but she and Avery manage to keep it up while Owen tries to talk sense into him. Unfortunately Owen manages to get himself shot, just about a the same time that Mere comes in and tries to get Gary to shoot her instead, promising that she's important to all three of his targets (Derek, Lexie and Richard) and that would hurt them more than shooting them. Cristina and Avery instead put their hands up and let Derek bleed out, and Mere falls to the floor screaming while Gary drifts out... then Avery reattaches all of Derek's monitors. Fakeout! Cristina orders Mere to work on Owen while she works on Derek, and Owen turns out to be fine as the bullet just ("just") went in and out cleanly through his upper chest/shoulder. There are a couple of tense moments, but Cristina manages to save Derek's life. Unfortunately it's all too much for Mere, and while she's working on Owen she also has a miscarriage, because there hasn't been enough going on already, but ultimately she's okay too.
Finally, Richard can't stand waiting outside, especially after he's treated to a montage of 911 calls from all of the doctors that he loves like his own family. He gives the cops the slip and goes inside to find Gary waiting in the same room where his wife was unplugged, but Gary doesn't shoot Richard immediately because he admits he only has one bullet left. See, he wanted to bring his flask because he thought he might need a drink, and so he wound up without enough bullets to finish the job he came for, which was to kill all the docs he held responsible and then finish himself off. He offers Richard a drink and a couple of times but Richard manages to be strong and winds up pouring the booze out while telling Gary that it's not worth it to shoot him, because he's content that he's had an amazing and full life. He points out that if he shoots Richard and not himself, he'll rot in jail, but if he uses the bullet on himself he will get to go be with his wife. As the SWAT team surrounds the room we hear a gunshot but don't see until a few moments later that Richard is totally fine, and we wrap up the season with a discombobulated Mere throwing out her positive pregnancy test on the way to go visit her recovering Post-It husband.
I realize that a montage of events will be very handy when they aren't airing these episodes back-to-back in repeats and syndication, but considering we go directly into hour two without even a commercial break, it's kind of funny to see a recap of what we saw 15 seconds earlier: Derek getting shot, April freaking out, and Meredith falling apart while Cristina tries to hold her back. As a consolation for taking a bullet to the chest, Derek gets to do the final voiceover of the season: "The human life is made up of choices. Yes or no. In or out. Up or down. And then there are the choices that matter. To love or hate. To be a hero..." Here we are treated to a brief shot of him bleeding out, just so we know for sure that he is in fact the hero. "...or to be a coward." Now we get a quick shot of Gary and his screwed-up angry face, just in case we didn't know that shooting everyone at the hospital where your wife died is in fact a cowardly thing to do. "To fight or to give in. To live, or die." Bum bum bummmmm.
But Derek isn't dead yet -- his eyes fly open and he begins to panic a little as he looks at the blood on his fingers and realizes that he's been shot. April runs up to help him so Gary points the gun at her, and in a crazed, panicky voice she begins to stutter all sorts of personal info about herself: She's 28, she's from Columbus, Ohio, her mom is a teacher, her dad a corn farmer, and she's the second of four sisters. That said, she adds for good measure that she's barely lived, and no one has loved her yet, and she pleads, "I'm a person." As odd as this all seems, it does the trick, and Gary has tears in his eyes as he orders her to run, his tone of voice saying he might change his mind if she doesn't leave immediately. She doesn't have to be told twice, and after she sprints off Gary points his gun back at Derek, who is struggling for breath and trying to talk to him. Just then he hears a sound and looks down to see the SWAT team coming in, guns drawn. Though he has a crystal-clear view of them and in fact is on a catwalk in plain sight, I guess they don't actually look up and see him, but it's all for the best of the story as we still have about 59 minutes of the episode left. As he runs off, Derek VOs, "Live or die. That's the important choice. And it's not always in our hands."
Cristina seems to have gotten Mere back into the supply closet, and all I can think about is how that place must smell badly like vomit right now. Meredith wants to get out in case Derek is still alive, but Cristina points out that she could get herself shot, and tells Mere that she won't let that happen. "You want to go, you're gonna go through me." It's an impressive declaration but she's no match for a twiggy girl whose Post-It husband is bleeding out a short ways away, and Mere throws her out of the way and face-first into a supply rack before running out. I have no idea how Cristina didn't break her nose after that, but she seems all-in-all pretty intact and just dazed.