A three-carriage tube train eased into the abandoned Brompton Road station and the doors opened. It was indistinguishable from standard Tube stock, aside from the shining gold livery on the handrails and a seat moquette featuring a dragon, a unicorn and the house colours of Hogwarts. The classic roundel on the carriage doors was the same, the same lady voiced the tannoy announcements, and these trains were just as crowded as any on the world's oldest underground transit network.

The only difference was that these trains carried magical people, rather than their harassed Muggle counterparts.

Tube trains on the Piccadilly Line may not have stopped here for some time, but the Wizarding World had it's own line on the underground network. If it was on a map, it would be coloured in gold. It was called The Merlin Line and ran not only through central London, but connected to an underground rail network that spanned the entire length and breadth of Great Britain.

Harry didn't know this, of course, but there was even a secret connection to the line that ran straight to Annwn. If he so wanted, it would be quite the easy thing to take a trip back home.

But right now, Harry and Sirius were heading in the other direction. When the train arrived they hustled and hurried to get on, squeezing into a corner of the carriage where Harry could see one of the overhead maps, so he could track their journey. Next to the map was a poster for the latest Weird Sisters album, information about an upcoming Celestina Warbeck concert at the Diagon Palladium, and moving signs warning Londoners to Mind The Gap.

Then the train began to move off. A voice came over the tannoy. "The next station is … Knightsbridge. Change here for Harrods Department Store and Paul Daniels Magical Toy Shop. This is a Merlin Line train to Immore Alley." Harry watched out of the window as the train hurtled along, leaping past other trains and even shooting through crowded stations. Nobody seemed to notice them gunning past, but Sirius didn't seem too concerned, immersed as he was in a copy of The Evening Standard, that had been left by another commuter.

"Er … Padfoot," Harry began, using the codename Sirius has given for him to use when out in public. "How come nobody can see us going past?"

Sirius turned his ridiculously turbaned-head on him. He looked ever sillier today as he'd covered his eyes with a ski-mask, to make him look even less approachable.

"Well, Harry, Muggles don't notice magic unless they look really hard," Sirius began. "And even then they will try anything to disprove it. If they do spot anything, they explain it away as a trick of the light, or the wind changing direction, or other such nonsense. Some wizards even leave the magical world to become entertainers for the Muggles, just to see how far they can push the limits of Muggle disbelief."

"Really?" Harry asked, fascinated.

"Oh yes," Sirius replied, turning the page of his newspaper. "There is a famous magician in Las Vegas - that's in the USA - who regularly flies during his shows. He even walked through the Great Wall of China once. The Muggles all come out of his shows asking how does he do it? - but they will accept almost anything but the truth - which is that he actually does the things they see with their own eyes. Good luck to him, I say. He's made a fortune from a basic Levitation Charm. You'll learn how to do that in your first month at Hogwarts!"

"I'll make things fly?" Harry asked, then he remembered suddenly. "Oh, is that wingardium leviosa?"

Harry couldn't see Sirius' face, on account of the mask and turban, but he could almost imagine his eyebrows shooting into his coiffured fringe.

"How do you know about that spell?" he queried.

"I … sort of … did it," Harry explained shyly. "When my Dad was building the owl coop outside our house in Annwn. Ooh, that reminds me, I haven't replied to my Mum's letter this week, she'll be so cross with me! Anyway, my Dad was demonstrating the spell when he floated the roof onto the coop. I was trying to copy the wand movement and say the magic words properly, and I concentrated really hard and the roof just went up a little bit. Not much, and only for about three seconds, but I did it!"

He couldn't help but sound pleased with himself.

"Well, if that's the case, our little trip out today should be a doddle for you!" Sirius laughed deeply.

Harry grinned widely to himself. Sirius was taking him to the aerodrome, where he could start teaching him how to fly. Harry was bouncing on his heels with the excitement. Learning to fly … doing some actual magic! He couldn't wait.

All week he'd been peppering Sirius with requests for hints and tips, and information on Quidditch, the big sport in the magical world. In the end, Minerva got so fed up with the barrage of chit-chat around the flat, that she returned one day with a book she'd checked out of the Hogwarts library, called Quidditch Through the Ages, which was a fascinating read and had the dual bonus of shutting Harry up for a good few days.

There was only so much you could get from a book, though, and Harry was now eager to get on with the business of gaining practical experience.

Harry and Sirius left the Merlin Line at Fizzick Alley station and emerged into the sunshine of late August London. Fizzick Alley was a fascinating street and Harry quickly saw that it was a little bit like a magical sports village. There was a Gobstones Play Centre, little cafés full of angry Wizard's Chess sets eager to enter into battle, and a huge auditorium right in the middle of it all, that Sirius informed Harry was the Fizzick Duelling Arena, where this years' World Championships would be held just after Christmas.

"Do wizards actually duel then?" Harry asked in astonishment. "They fight with magic?"

"Yes. Witches, too," Sirius replied. "The current world champion is a French witch, actually. She has the ability to use two wands, you know, one in each hand … very rare skill, and very useful for casting Shield Charms with one and offensive spells with the other."

"Is it dangerous?" Harry queried.

"Athletic Duelling is highly regulated and only specialised sporting combat magic is permitted," Sirius explained. "But competitors do get hurt, it's just the nature of the sport. Deaths are uncommon, but just like in Muggle sports - such as boxing - these things happen from time to time."

Harry looked up in horror. "Wizards can die?"

"Of course we can. We do. We're only human, after all."

"But I thought … I thought magic would stop all of that," Harry muttered. "That would be the sort of thing it does, I thought."

Sirius looked down, grinning fondly at Harry's innocent naivety. "Magic can put a stopper in death, and some magic can certainly prolong life … such as aspects of alchemy … but no magic can prevent death completely. If the magical remedy was stopped, the user would die like anyone else. Even magic has it's limits."

"Can it bring people back to life?" Harry asked, curiously.

"There is no magic that can awaken the dead … at least, not in any form that you'd like to meet. Trust me on that"

Harry swallowed hard and shivered involuntarily, Sirius' loaded tone conjuring images of swarms of zombies and vampires sweeping over the horizon in Harry's harried brain. He had to get back to topic.

"So wizards can die, and some take the risk in the name of entertainment," Harry surmised. "Seems a bit silly to me."

"There is always glory to be found in sport, and singular combat sports are some of the oldest and most self-rewarding," Sirius answered. "Some may see it as archaic, but there is something in our nature that loves to celebrate a champion, and that leads to a yearning in many to become one, to earn that adulation for themselves. The promise of glory makes it worth the risk."

Then Sirius' expression darkened. "In some cases, such ambition to demonstrate power and dominance could lead down a very Dark road. So, we should be thankful that these warriors choose to fight their duels in a controlled arena, rather than facing each other in real life ones."

Harry baulked and came to a stop. "Real life ones?"

"Yes, real life ones," Sirius stated without ceremony. "An important thing to remember, kiddo, is that not all wizards are good ... some are bad, some are worse than that. It falls to us good ones to do something about it. After all, as the saying goes, evil flourishes when good people do nothing."

"So … good people duel the bad ones, to stop them doing evil?" Harry clarified in a solemn voice. Sirius nodded the affirmative. "Who are these good duellers?"

"They are highly-trained Magicals whose job it is to hunt and catch Dark Wizards," Sirius explained. "We call them Aurors."

"Auror," Harry repeated in reverent awe. "That sounds like an exciting life."

He rather thought he might have hit on a career goal with this revelation. Imagine being a Dark-Wizard Catcher … that might be rather something.

"It's certainly seen as the more glamorous part of magical law enforcement," Sirius agreed. "But highly dangerous too. Besides, killing Dark Wizards is an easy way out for them … bringing them to lawful justice is just as satisfying if you ask me. As much good can be done in the courtroom as on the duelling field, Harry. Remember that."

"I will," Harry vowed. Then he grinned up cheekily. "But Duelling does sound much more exciting, though."

"It certainly does! I'll tell you what, if you're a good boy, I might try and blag tickets for the Duelling Grand Final," Sirius went on gleefully.

"As a Christmas present?" Harry asked hopefully.

"Oh no," Sirius replied with a gleam in his eye. "I've already got you that. If fact, I think I might just give it to you early."

With that he reached into his jacket and pulled out what looked like a shiny, polished twig. Then he drew his wand, whispered a quick spell, and the twig was magically resized … to a slick, gleaming racing broom!

"Merry Christmas, kiddo!" Sirius beamed, offering the broom to Harry.

"F-for me?" Harry mumbled. "I cant … this looks very expensive."

"Oh it was," Sirius laughed happily. "But I'm rich as all hell, so don't worry about it."

Harry blinked at the broom. As much as he wanted to be modest and humble and carry on his weak protests, he really, really wanted this broom. He had fallen in love with it at first sight.

"Is it really mine?" Harry hushed in astonishment.

"It certainly is," Sirius grinned. "Top of the range, too. Not due to go on general sale for another few months. But I know a fella, who knows another fella, and he knows a bloke who went to see a man about his dog about it. I got it from him!"

Harry didn't understand any of that. All he was interested in was the burnished handle, the perfectly cut tail-twigs, and the words Nimbus 2000 that were stamped into the hand-grip. It was beautiful and even Harry, who had never seen a magical broom in his life before, could appreciate the craftsmanship of it.

"Well, are you going to make love to the broom with your eyes all day, or are we actually going to go and fly the thing!"

Harry nodded and allowed Sirius to guide him further along the street, towards a sky blue dome at the very end. It was huge and had giant moving projections on the brickwork of witches and wizards zooming about on brooms, of synchronised flight displays and, of course, highlights from famous Quidditch matches. Harry looked up and read the sign on the giant awning, which was in the shape of a massive golden ball with wide wings outstretched on either side.

"Hooch and Hardbroom's Magical Flight Centre," Harry recited.

"Or, as it's more commonly known: The Big Blue Tent!" Sirius derided.

"Who are Hooch and Hardbroom?" Harry asked, as Sirius opened the door for him.

"The proprietors of the centre," Sirius explained. "Rolanda Hooch teaches first-year flying lessons at the beginning of the Hogwarts term, and acts as referee for Inter-House Quidditch matches. She used to play Beater for the Holyhead Harpies team, you know. And Joy Hardbroom is an international champion in broom-mounted Air-Dancing and synchronised team flying. She works for Ethel Hallow's Witches Preparatory College in Jericho, Oxford when she's not here."

Sirius led the way through to the Reception Area. He and Harry deposited their coats, paid for use of the practice airspace, and made their way into the giant arena.

And Harry promptly forgot how to breathe.

For the place was enormous. Over to the left was a full-sized Quidditch stadium, a dome-like structure with enough tiers for forty thousand spectators. Harry could just about see the top of the central fifty- foot high scoring hoop. It wasn't in use today, but it was the home arena for the Farringdon Fliers, the local team who played out of London, and their club banner was fluttering merrily away on a large pole near the turnstiles to the stadium.

Off to the right were a series of smaller structures. These were little more than oval-shaped mounds, about thirty of them, and each mound was reserved for use by individuals or small groups. Harry quickly understood that they were practice patches, where new or inexperienced fliers could build up their confidence, before moving to a much larger space in the middle of the arena, which was awash with brooms and their riders, some doing tricks and turns, others creeping along and holding onto an invisible rail at the side.

All in all, it was like the magical world's version of an ice rink.

Sirius led Harry to one of the smaller mounds that was free at the far end. They entered through a door made of the same netting that divided each practice patch, and Sirius instructed Harry to put the broom on the floor, which he promptly did.

"Now," Sirius began, rolling up the sleeves to his garish purple shirt. "What we are going to start with today is basic broom command. This is just to see how adept you are at broomstick control."

"Okay," Harry nodded. "Is that important, then?"

"Of course," Sirius replied. "A broom is an enchanted object. It will interact with your own magic to complete a sort of magical circuit, if you like."

"Oh, I get it," Harry pondered. "So it needs a bit of my magic to work?"

"Exactly," Sirius confirmed. "And through your magic you can issue it commands. Such as to fly in the first place."

"Ahh!" Harry exclaimed. "So if a non-magical person picked it up, it wouldn't fly at all? It would just be like a normal broom?"

"Well yes, apart from the fact that it has a seat, dragonhide hand-grips, and cost about the equivalent of a high-powered Muggle sports car!"

Harry gulped. He didn't want to think about the price of the broom at his feet. He almost didn't want to try and fly it, in case it turned out he was utterly rubbish at flying and broke it at the first attempt.

Sirius seemed to sense Harry's reticent turned and grinned to try and reassure him.

"Dont fret, kid, we will just take one thing at a time. Up at Hogwarts, your entire first lesson will be about getting the broom to trust you, before you can even think about flying."

"And how do you get a broom to trust you?" Harry asked sceptically. He didn't think he liked the idea of a magically enchanted object that could think for itself. He rather thought you shouldn't trust something if you couldn't see where it kept it's brain.

"You have to command it," Sirius went on. "Be relaxed, not fearful. Feel the energy of the broom, offer yours out to meet it halfway - almost like you're bowing to it in respect - and then just say, UP!"

Sirius had held his hand over the broom, which jumped right into his waiting grip as he spoke the last word. Harry went wide-eyed at the display of magic before him.

"Wow!" Harry whispered. "That was brilliant!"

Sirius grinned at that. "If you liked that, then you're going to absolutely love this!"

And with that he flung his leg over the broom, kicked off from the ground and soared up into the air. Harry whooped in joyous excitement as he watched Sirius lean forward, then shoot off like a human dart in a fast lap or two around the practice patch. He returned to Harry then, dismounting with the grace of a gymnast and placing the broom at Harry's feet.

"Wow! That was amazing!" Harry crowed.

"Your turn then," Sirius beamed. "Merlin, I do miss flying. Now don't feel bad if you cant -"

"Up!" Harry yelled.

To Sirius' immense surprise, the broom leapt up first time. Unfortunately, it also caught Harry by surprise, too. The broom missed his outstretched hand completely … and whacked Harry hard in the face. His glasses took the brunt of the impact, but he did get a nasty wallop to the forehead for his trouble.

"That wasn't funny"!" Harry snapped crossly, rubbing at a nasty bruise that was blooming between his eyes, as Sirius doubled-up with laughter next to him.

"Are you sure?" Sirius panted through his chuckles. "You didn't see it!"

"Can you fix my glasses, please?" Harry frowned.

"Pass them here," Sirius replied, drying his eyes. He took out his wand and tapped them to the frames of the glasses, which were repaired in a puff of magic. "There … hold on … what's that?"

"What's what?"

"That!" Sirius cried, pushing back Harry's fringe to look at his forehead.

"Oh that," Harry replied, quickly brushing his fringe down shyly. "That's nothing."

"It isn't nothing … it's a scar."

"I know," Harry moaned. "My Mum she … did a sort of ritual with me. It produced a bolt of lightening, which hit me right there, where the scar is. I don't know why, but it wont go away. It's so bright it's like it happened yesterday. It's so ugly, isn't it? Do you know how to remove it, because neither Mum nor dad could?"

"No, and I wouldn't if I did," Sirius muttered, examining the scar closely. "Scars can be very useful, you know. Dumbledore has one above his knee that it is a perfect map of the London Underground. It came in very useful when the Merlin Line was being built. It helped the magical engineers avoid the other Tube lines."

Harry perked up at that. "Could my scar be useful like that, then, do you think?"

"It could well be," Sirius pondered. "Magical scars often have unexpected benefits. Do you know, looking at it closely, yours looks a bit like -"

" - the Sowilo Rune. Yeah, that's what my Mum said," Harry cut in with a huff. "The ritual was to see if I was an Alchemy Adept. Whatever that is."

"And sowilo responded? To the ritual?"

"Y-yeah," Harry stuttered. "Is that … good?"

"It could be very good," Sirius grinned. "It's certainly very interesting. Have you shown this to Minerva?"

"No, I'm a bit embarrassed about it, to be honest. I hate the scar."

"Dont be," Sirius cajoled. "A blessing by Sowilo is something to be humbled by. Just ask Minerva."

"Does she know a lot about this then?" asked Harry eagerly.

"A lot? She knows more than almost anyone!" Sirius cried. "She was the Ancient Runes Professor until this year. She wrote the textbook for the course!"

"Really!" Harry whispered. "I'll have to ask her about it later, then, no matter how ugly the scar makes me. But forget about all that for now … I want to try flying again. It's my turn to have a go."

He placed the broom down with purpose, held his hand over it again, and ordered the broom into his hand.

It went willingly … as though it were it's right to be there.

Sirius looked just as surprised this time as he did the first. Harry ignored his warnings about taking things slow and simply hauled his leg up over the broom, took a steadying breath for courage, and then kicked off from the ground with determined purpose.

And he rose with control and grace and a fierce sense of joy. This was easy, this was natural.

Harry swept upwards in a rush of fast air, grinning widely as his cheeks rippled and his hair flapped wildly as he soared high above a whooping Sirius down below. This was wonderful! He had found something he could do without trying! He could even twist and turn and make the broom do what he wanted with only the merest of thoughts about it.

And Harry was cheered immensely by the experience … maybe he wouldn't turn out to be such a useless wizard after all.

Harry zoomed around and around the practice patch for the best part of the next hour. Sirius clapped and cheered and encouraged, before he eventually hired his own broom so that he and Harry could race each other. But the Cleansweep Four was no match for the Nimbus, and Harry won every time.

But the time they left Hooch and Hardbroom's some time later, Harry was bright-eyed and alight with vigour.

"That was brilliant!" he sang, as they made their way back towards the train station. "Can we come again next week?"

"We certainly can, so long as you finish the tasks that Minerva sets you in good time. I don't want to get on her bad side!" Sirius chuckled.

"I promise I will," Harry vowed, spurred on by the motivation to fly again.

"Tell you what, next time we'll pop into Quality Quidditch Supplies first and pick up an equipment kit. I thought you might be the build for a Chaser, Harry, but you move so fast on that broom … you're a natural. I knew Charlie Weasley - best flier I ever saw - but I think you might actually be better."

"And was he a Chaser?"

"No … he was a Seeker," Sirius explained. "And I think we might try and focus on that for you. We'll do a bit of focused training … see if you can catch enchanted keys or something before we let you loose on an actual Golden Snitch. They'd be about the right size to practice. If you're any good, I wont have to bribe Minerva to let you onto the House team for next year, after all … she'll have no choice but to pick you. Besides, she told me in confidence that Gryffindor just played Slytherin last week and got absolutely flattened by them. And if I know Minerva, she will be just dying to get revenge on old Snivellus Snape next time round."

"I'll work hard. I really will," Harry swore. "I'll make it on to that team. I'll make you all proud."

"I know you will," Sirius grinned, ruffling Harry's hair so much like his father used to do. Harry allowed it, was comforted by it.

But then something happened that changed all that.

They were passing Knockturn Alley on the way back to the Merlin Line tube station, when Harry spotted a very dicey looking wizard skulking in the shadows. He was nervous, bobbing on his heels as he scanned the streets around him. He looked like he was waiting for something … or someone. When he spotted Harry and Sirius-in-disguise, he moved suddenly and blocked their path.

"Quirrell? Quirrell? Is that you under there?" the wizard heaved in a low breath. "I heard your costume had gotten even more stupider."

Harry expected Sirius to turn, inform the wizard that he'd gotten the wrong man, and they'd continue their stroll along the alley.

But he didn't.

"Yeah. It's me," Sirius growled in a tense voice. "What are you doing accosting me in broad daylight?"

"No choice," the wizard huffed, gruffly. "Got your orders here from our Albanian friend."

"Not here," Sirius hissed, batting the wizard's arm down as he reached into his cloak. "Someone will see."

"Ditch the kid."

"The kid is with me. He stays. We'll do this is Knockturn. Plenty of nooks and crannies down there."

The wizard nodded and led the way back to Knockturn Alley. It was dark and grim down here, the very air smelt of danger and underhandedness. Harry felt a bead of fear traverse every link of his spine, right the way down his back, the deeper into the gloom they went. Angular, mis-shaped buildings loomed overhead, blocking out the light, and suspicious eyes seemed to flash from every shadow. Harry didn't like this place at all.

Sirius then suddenly bundled the wizard into a narrow alleyway leading off the main street, before herding Harry to stand in front of the window to a shop called Borgin and Burkes. A disembodied hand reached out from a mahogany plinth, next to a flayed skull on an acid green cushion. Harry shuddered as he considered them both, and what they might do.

"Stay here, and do not move," Sirius ordered. "Do not enter the shop for any reason."

"What's going on?" Harry whispered in a shuddery voice.

"I cant tell you now. Just stay still. I wont be long."

Then Sirius hurried across the street to rejoin the dirty wizard. The street was so narrow that Harry could still hear snippets of their conversation when there were no other passers-by. And what strange chatter it was too."

"This is the list," the wizard was saying, lowly. "If we can find just one of these things, we'll have a chance to bring Him back."

Harry didn't know why, but he imagined the word him with a capital H … as though they were talking about a deity … or a master of some other sort. Harry gulped hard at the implications of that.

"Looks random … what are these things?" Sirius breathed back as he cast his eyes over a scrap of parchment the other wizard was holding.

"Dunno … He wont say. Not likely to tell grunts like us, is He?"

"No, I suppose not," Sirius replied gruffly. He read the list again. "These notes, on the right-hand side … are they last known locations?"

The wizard nodded. "It's where He said He left them. A diary in the library of Malfoy Manor, but the allegiance and loyalty of that family is in doubt, so He thinks it's too risky to bring them on board just yet. A cup in the Lestrange vault, but they are tied up in Azkaban so we have no way to access it. There's also a locket that may or may not have been destroyed. We have people looking into that.

"Your mission is this diadem thing. It's at your school somewhere, in a Room of Lost Things. But the room itself is lost, so you have to try and find it. Then find the diadem, whatever that is."

"It's a type of tiara, you oaf," Sirius scoffed. "Elaborate jewellery isn't your thing, I see. What about this ring at the bottom of the list?"

"He wouldn't tell us anything about that," the wizard muttered. "Must be something personal. Said he wants to sort that himself once he was able."

"And that's it? Didn't say what any of these things were or why he wanted them?"

"Nope. Just gave the orders to find them. Then to burn the list."

"I'll do that," said Sirius. Then he drew his wand and placed it to the tip of the parchment.

Then he flicked an impish grin at Harry, who instantly knew what was going to come next …

… for in a split second Sirius flicked his wand from the parchment to aim at the wizard's head, while at the same time mouthing a word that looked to Harry's lip-reading skills like oblivion. There was a dull flash of light, the wizard's eyes went from momentarily enraged to completely dull and docile, and Harry realised he'd just witnessed his first offensive spell being cast.

Sirius then knocked the out wizard with a single, well-placed punch to his inert temple. The wizard crumpled to the floor and Sirius left him there in an awkward heap, but not before accidentally crushing the wizard's fingers where he stomped on them with his heavy boot as he passed.

"What did you do that for?" Harry hissed.

"Had to make it look convincing," Sirius replied, grimly.

"No, not that," Harry went on. "Why did you do any of it? I know you hit him with some kind of spell. What did he do?"

"I cant tell you that, and I forbid you to ask any more about it," Sirius retorted in a tone so firm that any further protest from Harry was silenced in his throat. Sirius squeezed his shoulder warmly. "It's for your own good, kiddo. Trust me. Now, let's get moving before this scene draws any attention onto us."

"Are you worried? About the Aurors?" Harry muttered as he dropped in beside Sirius' quickstep out of Knockturn Alley. Sirius gave him a weighty look.

"No, I'm not bothered about them," he hushed lowly in response. "It's whoever gave the orders to that hapless wizard back there, whoever he and Quirrell are working for, that's who I'm interested in. And this bit of intelligence is the best lead I've yet found to root them out."

Harry cried out in subdued triumph. "So that's why you're using this disguise! It's not just to stay hidden! You're pretending to be this Quirrell person so that you can intercept messages to him!"

Sirius grinned down. "You really are a bright young thing, aren't you? I'll have to keep my wits about me where you're concerned!"

"Or you could tell me what's going on," Harry offered in hope. "Maybe I can help."

"Nice try, but the best help you can be is to forget all about this and let us adults deal with it," Sirius chuckled back. "You stick to being a child, kiddo … there will be plenty of time for you to get into your own sort of mischief once you've grown up a bit.

"Now come on, I have to get you back and safe to Minerva, and then get this information to Dumbledore. Trust me, Harry … he'll know what to do …"