They ran for a coffin, the better to get her back into the sky -- the Doctor makes a quick detour back to now to get the door codes from old Scrooge -- heading down into the crypts. Scrooge went straight to the woman's coffin from fifty years hence, and the Doctor wondered subtly why she keeps cropping up in this story. "My name is Abigail Pettigrew, and I'm very grateful for Mr. Sardick's kindness... I could not have chosen this path were it not for the compassion and generosity of the great philanthropist and patron of the poor, Mr. Elliot Sardick, but I'm also surrounded by the fish, the beautiful, iridescent, magical fish..." She loves the fish, she spoke like that for quite a while, which is why Scrooge loved and loves her. The Doctor was again horrified by this whole human-collateral thing, and they agreed Scrooge Senior was a dick, and then the shark showed up again, down in the crypts.
There was the submarine beeps of the shark coming closer, the pieces of the screwdriver calling itself to itself, and then silence, and then Abigail, awake and singing the shark to sleep. Crouched in the mist, casket lying open, singing "In The Bleak Midwinter." The boys converged on her, staring, discussing quietly whether it was "the singing" or simply "notes resonat[ing] in the ice, causing a delta wave pattern in the fog," that calmed the shark and delights the fish. They kept biting the Doctor as he shouted science, and figured out how to save Amy in the future, but Scrooge finally helped him realize it never mattered. Whether Abigail's singing or the resonation of ice crystals, it helped.
The boy, the woman and the Doctor stood in the door of the TARDIS, staring out at the fish as they danced. Scrooge took out his camera and snapped pictures of her, smiling in the ice and fog; Scrooge retrieves the pictures from an old desk and stares at them, one by one. The Doctor opened up the shark's coffin and released her, out into the sky, but when he asked Abigail about the countdown lock on her coffin -- Eight, it says, eight left -- she was pretty cagey. "It pertains to me, sir, not the fish. You are a doctor, you say?"
The Doctor wondered about that, but not enough to ask. They locked her back up, and returned in a year's time for their next Christmas visit. This time wearing Santa hats. He produced a carriage and summoned the shark, and they rode through the sky, laughing, while the city slept, and locked her back up again. Scrooge looks at them, photograph after photograph, smiling as the years pass: Once with matching bowties, the next with bowties and fezzes, once a great bloody long scarf, visiting all of time and space. Abigail at the pyramids, Abigail in the Alps. Every Christmas Eve they'd throw open her door and shout, "Merry Christmas!" And she'd smile and call out, "Doctor!"