Leah hurries off after spending the night with Arizona. This thing they have going is supposed to be casual. That means no cuddling in the morning. Stephanie is back to work after Lasik eye surgery. After hearing that she walked into a wall, Richard puts the kibosh on her doing his operation. He will let one of the other interns cut him open though. In other news, a patient is brought in along with the man he mauled. The guy was shot during an attack where he tried to chew off someone's face. He grabs Leah and violently sinks his teeth into her neck. He leaves a tooth behind. Leah's open wound puts her out of the running for the Richard surgery.
Meredith's grant was approved which means she doesn't have a lot of time for butterfly wing repairs. That's not a new medical procedure. It's a Halloween emergency regarding Zola's costume. Good thing she married a surgeon. If Derek can piece together brains, how hard could a wing-sewing project be? Trick-or-treating is taking place at Mer's old place. Alex is letting her have a party there. Cristina is a little hurt she wasn't on the guest list. She cheers up when Owen shows her the mad biter guy's x-ray. All off his vital organs are completely backwards. This is something she's wanted to see since med school.
Cristina and Owen operate on Backwards Organ Guy. Things go south fast, but the patient stabilizes. Christina tells Owen that he needs to operate backwards. He needs to tell his left hand to do what his right hand usually does. This approach works. Owen later convinces Cristina to attend Meredith's party. In other news, Richard doesn't feel like Bailey is letting the interns learn, so he kicks her off his case. He wants Meredith to take over and apologizes for what he said to her after she made his medical decisions for her.
Jo is getting radio silence from Alex after her meddling regarding his deadbeat dad. The two of them must work together to treat a little girl with stomach pains. Alex doesn't care for the girl's father, but it's mainly because of his feelings about his own dad. He rails on the guy after his daughter goes missing from her hospital bed. The little girl has a condition where any rush of adrenaline could send her into shock. She could literally be scared to death. The race is on to find her.
A sick elderly woman with freaky eyes insists on seeing her doctor: Heather Brooks. Shane keeps losing the mysterious woman. It's almost as if she's a ghost. The woman staggers down a dark hallway where Alex's patient is trick-or-treating. The frightened little girl collapses, as does the elderly woman. Alex finds them. Both patients are rushed to surgery. The little girl will be fine. Jo comforts her guilt-ridden dad and Shane opts to skip doing Richard's patient to stay with the elderly woman. He lets her know that he's her doctor now. He'll try to be as good to her as Heather was.
Jackson and April work on Victor, the man who had his face bitten. The guy fears he's been quarantined because he's becoming a zombie. April authoritatively lets him know that's not the case. In other news, Ben wants to have a special Halloween with Bailey and Tuck, who wants to be a mad scientist. He helps Derek suture Zola's butterfly costume. They even make a few special modifications. Their fancy stitching gives Derek an idea of how to put a fan inside the brain. He's not missing surgery at all.
Stephanie treats a woman with maggots growing inside her leg thanks to an untreated wound from four months ago. The patient freaks out. She does not like hospitals or doctors. Meredith is able to ease her fears by getting her to talk about past Halloweens with her now-adult son. The patient pulls through her surgery and convinces Callie not to short-change herself of the chance at Halloween memories just because of her issues with Arizona.
In random updates, Ben does up the house big time to celebrate Halloween. Bailey can see how happy Tuck is. She wishes it could be this way all the time. Ben says it can because he quit his residency. This disappoints his wife. Arizona catches Leah crying because of a fear of repercussions of getting bit. She tells the tale of a similar infection scare from her past. This helps and Leah will be just fine. Jo wins Richard's surgery gig and gets advice from Meredith that she can't let Alex push her away just because he hates being vulnerable.
Meredith's Halloween party is a big success. Everyone has a great time. Well, everyone who stays, that is. Cristina drops off cupcakes, but sneaks off before anyone sees her. She ends up letting Shane buy her drinks at the bar. Back at the house, Alex shuts off the lights to watch a scary movie. He's done with trick-or-treaters for the night. DING DONG! Someone's at the door. It's not kids though. It's Jo dressed in a fairy costume. She apologizes for what went down with his dad. Alex quickly shuffles her inside before the trick-or-treaters figure out he's home. Looks like these two will have a happy Hallowen after all.