And the Doctor dances away, and appears at the window behind the boy. The babysitter won the lottery, it seems, as well. He jumps on the bed and says hello to the camera, to the future: "Now, your past is going to change. That means your memories will too. Scary, but you'll get the hang of it." And as it begins to happen, it happens.
The Doctor swept round the room, talking about what was under the bed and in the cupboards: "Do you know, there's a thing called a face spider. It's just like a tiny baby's head with spider legs, and it's specifically evolved to scuttle up the backs of bedroom cupboards..." I think that's when I would have begun to trust him. He showed the boy the psychic paper: "I think you'll find I'm universally recognized as a mature and responsible adult?" But no, he'd broken it: Finally, a lie too big.
The Doctor and the boy discussed the fish, the fogs, taming the sky and like that. He promised to let the boy see the fish, and hung out his screwdriver while they hid in the cupboard, as a shiny bait. (No face spiders that time of night: "They'll all be sleeping in your mattress.") When the Doctor asked why the boy wanted to see the fish, he said it was because they were scary. A good answer, and the Doctor agreed, but it wasn't the real answer. During the last big fog the fish had broken through, a whole shoal of them. Nobody got hurt, but it was scary and everyone was there and they all saw, except for the boy. He didn't get to be a part of the story.
As the Doctor goes to check his bait, both Scrooges cry out. "Eyes on the tie. Look at me. I wear it and I don't care. Trust me?" They do. "That's why it's cool." That's what dorks always say, though. The Doctor head out into the dark room -- somehow Scrooge remembers this part even though it hasn't happened yet -- and before you know it he'd been attacked by a huge CGI flying shark. I won't lie, it looked awesome. Hanging there in the bedroom, swimming right at him. (If you're wondering why there was just this one shark and the rest of them fish, I can't tell you. Nor why it will still be around in fifty years, all alone.)
The Doctor dove back into the closet with his young friend, and they babbled at each other for a while as the shark attacked, having eaten a fairly large portion of the sonic screwdriver in the process. The shark crashed through the door, and old Scrooge loses video, and then both he and himself yelling about how the shark was going to eat them. Which: You're standing right there, it didn't eat you. The fun of writing about time travel is, one would think, actually writing about time travel. Whatever. So they escaped somehow, CGI anyway, and then the shark was lying out on the patio, unable to get home. And Scrooge wept for her, this stranded lady, and Scrooge weeps for her, this lonely hungry thing. "She was trying to eat you," the Doctor said tenderly, and they both knew that didn't matter at all.