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!"
Until the day Kazran was big, nearly her age, grown into almost a man, blushing and retiring. Beautiful. That Christmas, Abigail asked for something special: To be a ghost of Christmas Present. Three left, that time; their fifth Christmas, the three of them. She stood outside her family's house, weeping in the snow, as the Doctor watched. He recognized her sister, from when she will be older and more heartbroken. Kazran came and she took his hand. For a moment, she caught him blushing. And then who should appear in the window but the Doctor, ushering them in from the cold.