It was Boxing Day, and Harry had just woken up by James dashing into him on a miniature broomstick. He left Ginny to take James back up to his room, and rubbing his stomach where James had flown into him, went down the stairs where breakfast was waiting for the whole family.

Unfortunately, so was the Doctor.

He was waiting at the corner of the stairs, his shadow thrown across the wall, his face half hidden in darkness. Harry smiled, but it faded as the Doctor simply glared at him. Was this the same talkative extrovert who dashed about snatching dark chocolate?

"Doctor. What's happened." Harry looked behind him making sure no one was coming out soon.

"I've found out a few things." The Doctor stepped forward into the light, and he seemed to tower over Harry, even though Harry was slightly taller. "For instance, did you know that Azkaban wasn't meant to be a prison?"

Harry frowned "What-" But he was cut off as Ginny came down carrying James. The Doctor stepped back into the shadows and Ginny just walked past him, saying "Well, what are you waiting for? You know mum always loves seeing you open presents."

The Doctor smiled mirthlessly as Harry bewilderedly looked at Ginny, then looked back at him "Perception filter. You can see me because I let you. Ginny couldn't because I didn't let her. Anyways, Azkaban wasn't meant to be a prison. It was meant to be a way to control the Dementors."

Harry frowned. "Look, can we do this somewhere else? It looks really weird if I'm just standing here talking to myself."

The Doctor raised his sonic screwdriver, and the whirring noise of the TARDIS could be heard. It suddenly materialized around the both of them, then left immediately.

Harry looked around surprised. He had been in here before, but he hadn't had the time to study the vehicle. Amy came out of an adjoining corridor, rubbing her eyes "Doctor, there you are. Rory's in the kitchen making something, do you want to-" She broke off, looking at Harry. "He isn't supposed to arrest us for breaking out, is he?"

Harry rolled his eyes "No, I'm not. The Doctor was telling me something and when I asked to go somewhere else he just summoned his TARDIS." He turned to the Doctor, but he was leaping down the corridor yelling out at Rory to hurry up with the breakfast. Harry was stunned at the sudden change in character when he came into contact with his companions. He didn't want to think about what would happen if he lost them and there was nothing left to stem that ruthlessness and cruelty.

Harry followed Amy to the kitchen, marvelling at how enormous the TARDIS was. The biggest Harry had heard of was only three stories high including the jacuzzi, but by the looks of things the TARDIS was at least ten times larger.

"How big is this thing?" He asked Amy

Amy smirked "I thought you had this kind of thing too."

Harry raised an eyebrow "At least we don't need a noisy police box to travel around in."

"Oh yeah, you just fire a gun whenever you appear out of nowhere." Amy retorted

They had reached the kitchen, and the Doctor slid about on the smooth floor, grabbing things from the fridge and completely annoying Rory "Amy, could you get the Doctor to stop." He said as he fried some eggs.

Harry raised his voice over the oil hissing "Doctor, how big is this thing?"

"Infinite!" The Doctor spun around "That's what you get when you bring a fourth-dimensional object into a three-dimensional world. Well, not really four-dimensional or we'd create a wormhole in our wake as the bulge in the universe tried to correct itself, but..."

Harry wished he hadn't asked as the Doctor added to the general noise with his explanation of multi-dimensional engineering. Amy patted him on the back "You'll get used to it. Take a seat."

Harry sat down, not wanting to think of the reaction might have if he went missing on boxing day. Soon enough, the Doctor sat down and dug into the eggs Rory had made.

"Incredible things, the Dementors." He said.

Harry looked up sharply as the Doctor continued "I was wrong. They don't feed off negative energy, that's just what they produce. They need the good emotions: hope, joy, clarity, morals. Negative emotions create Dementors, but too much and the Dementor dies. So, they need the good emotions to balance it out."

Harry was looking at him wide-eyed "How do you know all this?" At that, Amy and Rory looked at each other, and the Doctor's energy diminished slightly. "I almost got sucked out. The Dementor was on top of me, and in the process, part of his mind went into mine."

Harry's mouth opened slightly "But, how are you talking, moving, living?"

The Doctor's face darkened and Harry wished he hadn't said anything. After a moment, the Doctor's face brightened "Well, enough of that. Now to Azkaban. Someone in the ministry obviously knew about the Dementor's anatomy or else it wouldn't have been established. You don't, of course, but someone high-up does. You see, each person has a finite amount of hope they can produce under the pressure of all that negative energy of the Dementors."

Harry cut in "Does Kingsley know?"

The Doctor shook his head "No, I plan to pay him a visit after this. Anyways, the ministry made Azkaban to stick the Dementors. It's like their nest except for they're starved the moment they're born. All the despair gives birth to a Dementor, but Dementors die just as quickly because they can't get enough positive energy. The influx of prisoners are just enough to keep the Dementors from dying out too quickly"

Harry couldn't believe what he was hearing "Are you telling me that Azkaban was set up simply to control the birth rates of Dementors? And they used human souls to do this?"

The Doctor nodded, and Harry could suddenly understand why the Doctor had seemed so grim when he had confronted Harry that morning. "Why are you telling me all this?" He asked.

The Doctor went to put his dishes in the sink "Because I wanted to know that you had no idea what was going on. I couldn't believe it when I found out just now, but if you knew and just let it pass..." He trailed off.

Harry nodded, but frowned suddenly "Wait, just now? You were talking to that Dementor last night."

"Well, if you could call it talking" the Doctor said "This is a time machine. I just went forward in time to this morning to get you. I needed to know."

Amy seemed happy seeing Harry surprised again and again. "Yup, get used to it. The Doctor's going to drop you off back at the Burrow, right after you left. Try to get in without being seen, it might take a bit of explaining."

The Doctor sprang up "Right, as Amy said, time to get you back home." And he dashed off to the control room.