"You're not Santa Claus," Rose said, sitting up in bed. "And I'm going to scream for mummy and she'll come and hit you and then the police will take you away and put you in jail."
"If I'm not Santa Claus," the Doctor said, "then why have I brought you this present?"
It was a large, badly-wrapped present. Rose scrambled to get at it and fell out of bed.
"What's that?" Jackie called.
"Oh, bugger," the Doctor muttered, and walked into a large blue box, which vanished.
Safely inside, he ticked off another sort-out-to-prevent-temporal-anomaly item from a very long list.
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