Chocolate Frog cards were cooler than conkers, more fun than football stickers, and more magical than any playground pasttime that Harry had ever encountered at primary school. It was nearing the end of the Christmas holidays, and he and Ron were lounging in the Gryffindor common room, squashy armchairs all to themselves, gorging themselves sick on Chocolate Frogs while playing game after game of wizard chess. Ron's mum had sent him several boxes of frogs, and Harry was trying to improve his card collection.

"Shouldn't we be trying to find out who Nicholas Flamel is?" he asked tentatively, moving his knight on top of Ron's pawn. The knight hefted its sword, sent it in a unstoppable downwards swing and smashed the helpless pawn to pieces.

Ron groaned and brushed the dust off the board. "We'll let Hermione deal with it when she gets back. She's the know-it-all."

Harry didn't think he could bear another one of Hermione's lectures. He could hear her voice already, bemoaning the way the two of them never did any work. With a sigh, he peeled the wrapper off another Chocolate Frog and watched as a white bishop brought a mace careering down on top of his black rook. The frog jumped up onto the back of his hand and scuttled up his arm before becoming still. Harry poked it to make sure it was properly frozen (he'd had a still-live frog before, and hadn't much enjoyed the sensation of it wriggling around on his tongue), before picking it up and popping it in his mouth. He flipped the card over, hoping it wasn't yet another Agrippa, and almost froze himself.

Staring up at him from the little pentagonal picture, was a figure with spiky brown hair, big eyebrows and wide brown eyes. He was wearing a trenchcoat and a knowing smile.

"Hey! I know him!" Harry exclaimed incredulously.

Ron looked up from his own frog. "Who?"

"The Doctor!"

"Oh yeah. He's an explorer type. My Mum fancies him."

"But he's not a real wizard. He's just a fictional character, isn't he? He's on Muggle TV."

"No, he's a real wizard. He's done some amazing stuff. Time-travel, space exploration, the works. What's TV?"

Harry ignored Ron's question. He was still staring at the Doctor, amazed that someone fictional in the Muggle world could be a real person in the Wizarding world.

"But he's an alien! He's got two hearts, and a time-machine that looks like a police box!"

"Two hearts? The time-machine is just a kind of shed with an undetectable extension charm put on it."

"What about the sonic screwdriver?"

"It's a wand."

Harry was baffled. "Well no wonder Dudley doesn't like Doctor Who. He hates everything to do with magic."

"There you go then. By the way, it's your turn."

Harry deftly moved his queen to take Ron's bishop, then realised he'd left his king wide open, with the white queen not too many moves away.

"Should've taken my knight I reckon," Ron said, as the black shards showered the board.

"So how does he explore, the Doctor?"

"The police box is a special type of portkey, I think. He's really good at teleportation charms, and he's done lots of research into time as well."

"And he regenerates!"

"He's a Metamorphmagus. A person who can change what they look like."

Harry couldn't believe it, but he knew it had to be true. He wondered whether Hermione might be as amazed as he was to learn the news. A classic science-fiction character in Muggle homes, but a famous Wizard in reality. But Hermione probably knew it all already like she always did.

"Well, at least it's not Agrippa!" he grinned.

"Check," said Ron.

Harry moved his King to one side and Ron moved to block him off. "And checkmate."

"No wait! I take it back!"

"It's done now, mate. No going back unless you can travel back in time. Can you?"

"No."

"Well then."

"But..." Harry grinned at his best friend. "Now I know somebody who does."