Okay, I'm a bit nervous about this chapter but I think it's a fairly decent explanation bearing in mind the following.
1. Canonically nobody alive (except maybe Voldemort) knows the full story of what happened that Halloween in Godric's Hollow.
2. Canonically nobody (including Voldemort) appears to have an explanation of how Harry and Voldemort ended up linked together.
3. I'm not a great believer in the Lily's sacrifice saved Harry theory. In my opinion Lily was doomed the moment Voldemort found the Potters in Godric's Hollow. His 'Get out of the way you silly girl' comments seem to be annoyance at another Potter getting in his way, not a genuine offer to spare her life. (Though I like whoever's idea it was that Lily was tricking him into a magical contract) If Voldemort had even slightest interest in keeping her alive he could have stunned her instead of using the killing curse.
"You ready Pup?"
"Been ready ages," said Harry looking up from a book on Apparation he found on the bedside table. He put the book down and stood up, grabbing is firebolt.
"You can leave that here," said Sirius.
"It would probably be safer."
Harry considered that for a moment and then begrudgingly slid the broom under the bed, pulling the blanket down to hide it.
"Okay come here and let me look at you." Sirius straightened Harry's t-shirt. "You'll do."
"Sirius," tutted Remus, gazing in disapproval at rumpled clothes Harry had worn yesterday. He flicked his wand in an elegant figure of eight.
Harry yelped as clothes straightened and stiffened.
"You always put too much starch in it Moony."
"Only if I wanted to look like I'd slept in my clothes. Now we better get downstairs before they send up a search party."
As the descended the stairs they me the Weasley contingent leaving the kitchen. For a moment nobody said anything.
"You haven't had any breakfast," said Mrs Weasley a bit desperately.
"It's okay Mrs Weasley," said Remus, "We'll eat later."
"And we really shouldn't keep Dumbledore waiting. He's a very busy man."
"Yes, yes of course."
Sirius, Remus and Harry hung back and let the Weasleys precede them into the meeting room. Dumbledore was already there, talking to a elderly, rotund Wizard in formal robes and half-moon spectacles.
The Weasleys were all clumped together around Mrs Weasley. Sirius took a seat a short distance away and pulled Harry down beside him. Remus took the seat on Harry's other side and they both positioned themselves so that Harry was half hidden from the room.
Dumbledore tapped his wand against the table, making a clear ringing sound to draw their attention.
"Good morning everybody, it is kind of you to join us."
"As if we had any choice," muttered Sirius, quite audibly. Dumbledore ignored him.
"If I may introduce my companion Gervase Philpott."
Hermione gasped. Sirius growled.
"Yes Miss Granger, the Head of the Department of Mysteries."
Looking across the room Harry could see Hermione was talking very low and very fast at Ron and presumed she was listing this Philpott's achievements. One of which appeared to be annoying Sirius and Remus judging from the way they were glaring at the man. Harry decided he didn't like Philpott at all.
"We should be very grateful to Mr Philpott for taking time out from his busy schedule to meet with us today." Dumbledore glanced at Sirius and apparently decided against another dramatic pause just then. "I am sure you all have questions about what happened that Halloween night and the days that followed."
"Yeah," Harry piped up, "Questions about trials and the lack of them."
Dumbledore straightened his glasses and stared directly at Harry.
"Mr Potter, as Mr Black is no longer your godfather, would you cease being so assiduous in cultivating the man's bad habits."
Harry opened his mouth to say he'd never much fancied smoking anyway, but Dumbledore didn't give him the chance and kept on speaking.
"Mr Philpott is here to help me answer your questions. Now would anybody like to start? Miss Granger perhaps?"
"Uh yes Headmaster, Mr Philpott." She stood up and Harry was unsurprised to see her pull a thick roll of parchment. "I have several questions. If Ron is Harry and Harry is Ron, how come Harry has Voldemort visions, and how come Harry is the one who speaks parsletongue and what about…"
"My dear child," said Philpott, "If I can stop you there."
Hermione sat back down with an annoyed little huff.
"I believe," he continued, "That many of your questions share the same answer. When my old friend Dumbledore asked for my help in hiding the Boy Who Lived, I was only too delighted to assist. Our first task was to try and comprehend exactly what happened that night at Godric's Hollow.
"You Know Who had apparently been defeated by a one year old child. Naturally this required some explanation. The most obvious was that Mrs Potter's death had activated a very ancient blood magic, although I have to confess we could not work out exactly how she achieved this."
"And I bet that burned, didn't it?" heckled Sirius, "Having to admit you'd been out-thought and out-magic'd by a Mudblood."
"There is no need for that sort of language Mr Black," said Dumbledore reprovingly.
"Oh, so when Philpott here turned down Lily's application to join his department it wasn't because she was a filthy Mudblood, my mistake."
"I really don't know what you're talking about Mr Black," said Philpott shuffling his sheaf of papers nervously.
"That's because Sirius has got it wrong as usual," said Remus, "You turned Lily down because 'you didn't need a wretched Mudblood's grimy fingers ruining decades of priceless research'. It was when James complained to the Ministry that you said you didn't see why he was 'making such a fuss about a filthy Mudblood'."
"Oh yes," said Sirius snapping his fingers, "I remember now. And James hexed him, remind me Moony…"
"Green and silver," supplied Remus.
"That's right. Green and silver all over. And, was it feathers or scales?"
"Scales for skin, feathers for hair."
"That's it. And this Mudblood you were too full of yourself to employ, she was the one who defeated Voldemort and your precious department can't even figure out how she did it. Yeah I bet that burns."
"If you are quite finished Mr Black," said Dumbledore repressively.
"Oh I've barely even started, but I'll let old Philly dig himself a little bit deeper for the moment." Sirius grandly waved one hand to indicate Philpott should continue.
Harry snuck a peek at Hermione. Ron had his arm around her waist and she was watching Philpott with wide betrayed eyes. Harry decided he really, really didn't like Philpott.
"If you would like to continue Mr Philpott," said Dumbledore. "And I will remind the floor that matters will move a good deal faster if we can dispense with the interruptions."
"Thank you Headmaster," said Philpott. "Ah," he frantically scanned his papers.
"Lost his crib sheet," said Sirius.
With a desperate gulp for breath Philpott found his place. "Here we are. From what evidence remained we were able to piece together some of what had happened. When You Know Who cast the Killing Curse at young Harry Potter somehow the Curse backfired striking You Know Who."
Sirius and Remus both sat up straighter.
"Due to a number of reasons which are classified at this juncture..."
Sirius started to say something but Remus grabbed his wrist and gripped it tightly.
"You Know Who had a significantly weaker soul than the average Wizard; it shattered under the impact and his physical body was destroyed. The majority of You Know Who's soul re-coalesced to form a Wraith which we now know took refuge in Albania. We believe some of his soul was lost for ever, and one piece lodged itself inside Harry Potter. Naturally we…"
"Wait a minute," said Sirius.
"Mr Black I grow weary…" began Dumbledore.
"Do you want to know how Lily pulled it off, or not?"
Dumbledore's indifference fell away. The gaze he fixed on Sirius was as intent as a raptor's.
"How?" he demanded.
"Just let me run through this and check I've got it right. Voldemort was struck by his own backfired killing curse. No spell ever touched Harry. You know this for definite?"
"Yes," said Dumbledore tersely, "The investigation revealed that James and Voldemort duelled outside the house but inside the only spell we could trace was Voldemort's use of the Killing Curse. He used it twice, less than a minute apart. As far as we could tell Lily used no magic that night and Harry was utterly untouched by any spell. Obviously that couldn't possibly be correct and I could only presume that in extremis Lily had accessed some long forgotten form of blood magic."
Sirius shook his head slowly, "You're a bloody fool Albus." He turned to Remus, "I am right aren't I Moony?"
Remus nodded, a small smile curving his lips.
"Yes!" shouted Sirius, jumping to his feet, "Lily you are a goddess among women and the true Queen of the Marauders." He swept into a courtly bow so low his bent knee hovered only an inch above the ground and it seemed to Harry his forehead skimmed the floor.
Glancing across at Hermione and Ron, he realised that, in a reversal of their usual roles, Ron was explaining the gesture to Hermione.
"It's a full pure-blooded ceremonial bow," said Remus to him quietly, "I've never been able to pull it off. If you don't start practicing by the age of five or six your muscles never quite adjust to it."
"Sirius," snapped Dumbledore.
"You do realise that you could have know at anytime if you'd just told us the full facts instead of blathering on about sacrificial magic. Hell, if you'd just told Remus the full facts, but oh no, we couldn't possibly have that."
"All right Sirius, you've made your point."
"Have I, have I really? Didn't your friend Philpott just mention facts that are classified at this juncture."
"You've made your point, Sirius."
"I hope so, because if you keep playing a lone hand, you'll end up playing on your own."
"Remus as you also seem to know the answer, perhaps you would be willing to share it with us."
"Oh no Headmaster," said Remus demurely, "Sirius has so few pleasures stuck here in this house I won't deprive him of his fun."
"Are you sure you want to know?" asked Sirius, "You've sold everybody this lovely romantic story of a mother's sacrificial love that's worthy of a ballad and an epic poem or two, but the truth, as it usually is, is much prosaic."
The room grew several degrees chillier.
"Tell. Me," ground out the Headmaster.
"Okay, okay, keep your robe on. And actually, now that I think about it, it is a story about love and sacrifice." Sirius' attention flicked briefly towards Ron. "It's about a man, the epic poem version always seems to forget James and I really don't think it's fair, a man who loved his wife and son so much he bought them the time they needed with his life. And while her husband died for them, the woman had the chance to make one desperate gamble. She knew it would cost her her life but she didn't care about that as long as her son survived.
"Unfortunately for romance aspect, the couple in question weren't a noble gentleman and lady, they were a couple of pranksters. So the heroine of the Wizarding World saved her son's life with a joke product she and her husband had invented six months earlier with a couple of their friends."
"Ahh," sighed Dumbledore, relaxing, "That's how you know. You recognised the effects from our description."
"Yes," said Sirius, "The simplest answers are usually the most likely. It took us ages to get the hex just right and make sure no trace of it would show up when people tried to work out what had happened. I don't really know why we bothered, we certainly never had any plans to use it. I guess we were so sick of Voldemort and his Death Eaters it was nice to have something else to talk about."
"We never even came up with a name for it," said Remus reminiscently.
"I think Potter's Ploy has a nice ring to it."
"Mr Black," said Dumbledore.
"Sorry. It was a simple thing really. The fiddly bit is enchanting a tiny needle of pure electrum with the hex. Then all you have to do is get the needle within an inch or so of someone's wand and it will stick as if it was magnetised. The wand will cast one spell normally to throw off suspicion, but the second spell will backfire. An enhanced version of the spell will travel back up the wand and strike the caster, kind of like how you described Ron's wand acting with Gilderoy Lockhart, Harry.
"James must have got Lily enough time to find one of our prototypes and she somehow managed to get close enough to Voldemort to booby-trap his wand. After that all she had to do was make sure the first Killing Curse hit her, not Harry."
"So that's how it happened."
"Uh huh, and how the Marauders defeated the finest minds in Wizarding Britain." Sirius sat down triumphantly. He nudged Harry with his elbow. "Proud of your godfather and godmother, kiddo?"
Harry nodded his head fiercely.
Dumbledore shook his head slowly in apparent disbelief, "I always said you were some of the finest students I ever taught."
"Unbelievable," stuttered Philpott, "Fifteen years of my department's valuable time wasted trying to figure out a Mudblood floozy's cheap trick."
There was a roar of protest from the assembled Weasleys but Sirius and Remus were already on their feet, wands out. A violent flash of green light seared Harry's eyes. When he could see clearly again Philpott had been transformed. He had the mottled scaly skin of a green and silver snake, while a great plume of cockatoo feathers stood up from his head.
"That was for James," said Sirius viciously, "This one's for Lily." Sirius' wand sparked and there was a low booming sound but Harry couldn't see any immediate effect. "She invented it especially for you but was too nice to ever cast it, said something about you not being responsible for your cultural upbringing. Fortunately she told me about it and I don't give a hang for your cultural upbringing."
"Sirius, you didn't?" said Remus.
"Yep. And why not? I always thought it was a shame to waste such a great jinx."
"I suppose it was at that."
Up at the head table, Philpott yelped with dismay.
"Figured it out have you Philly."
"Albus I must protest at my treatment at the hands of these ruffians. I accepted your assurances that Mr Black is an innocent man but perhaps I should investigate the matter further." He wiped his fingers fussily down the front of his waistcoat, leaving streaks of dirt in their wake. Noticing everyone's attention was fixed on his stomach, he glanced down and cursed.
"While you're in my house you'd better keep your grimy fingers to yourself," said Sirius, "And if Dumbledore didn't make you swear an oath of secrecy before bringing you here, he's not the man I thought him."
"Duty to the state overrides all other oaths," said Philpott, his loftiness somewhat marred by the fact he was holding his hands awkwardly out in front of himself as if they no longer belonged to him.
Remus coughed apologetically. "I think you might find that doesn't necessarily apply to oaths made to Dumbledore, Mr Philly."
Philpott's eyes flew to Dumbledore. "Albus is this true."
"I certainly wouldn't advise talking to anybody about these events Gervase. It might have rather unfortunate consequences."
"And you think people won't notice when I show up looking like this?"
"I'm sure all this unpleasantness will have been sorted out by then Gervase. But perhaps it would be better if I took over answering the questions for now."
"Fine." Philpott glanced around but didn't seem to find any sympathy. Folding his arms, he thrust his fists into his armpits and stumped off to a seat at the back of the room.
Dumbledore serenely took his place at the behind the table.
"Don't think for a moment I used up all my annoyance on your patsy, Headmaster," barracked Sirius.
"Ahh Sirius my boy," said Dumbledore, "What a politician you would have made if you could only control your temper and your desire to shock."
"You know funnily enough, my mother once said much the same thing."
Harry didn't need anyone to tell him that was meant to be an insult.
Dumbledore sighed, "Sirius please remember we are on the same side here."
"My enemy's enemy is my friend, huh. Depends how you define the enemy though, doesn't it?"
Dumbledore looked genuinely grieved. "Is it that bad my boy?"
He jerked his head towards Harry, "You can honestly ask me that with a straight face?"
"He miscalculated Padfoot," explained Remus, "He thought Harry no longer being James' son would make you drop him, not hold on tighter."
Sirius gave a disgusted snort and settled deeper into his chair. Dumbledore stared at him with an arrested expression.
"Questions," prompted Remus.
"Right," said Dumbledore pulling himself together, "Questions. We haven't really addressed Miss Granger's questions yet have we? Knowing how Lily Potter defeated Voldemort does not change the essential facts of the case. Voldemort appeared to be gone for ever but our scan of Harry Potter revealed a piece of Voldemort's soul lodged within him and I feared more of Voldemort was likely to have survived. That meant it was essential to guard the Boy Who Lived from further attacks. I still believe the method I chose was the best under the circumstances. But I fear on this we will always disagree, so I will move on.
"Swapping the children was relatively simple. One infant of less than two years looks much like another. A couple of renaming ceremonies and a few hereditary charms to ensure the children grew up resembling the family they had been transferred to…"
"You're skating Albus," Sirius pointed out.
"There is no need to go into the details at the present moment."
"If you say so." He didn't sound convinced at all.
"I do," said Dumbledore with gentle reproof. Sirius rolled his eyes.
"But," Dumbledore continued, "There was the matter of the piece of Voldemort's soul. I was loath to leave it inside the child for fear of future complications but at the same time I could see occasions in the future when it would be useful to have such access. It was such a small fragment it was unlikely to have an effect on the child's character, or indeed any effect at all while not in close proximity to Voldemort.
"So I, together with three other Occulmency experts, including Mr Philpott here, extracted the fragment from Harry Potter and transferred it to Ronald Weasley."
"You never told us that," said Mrs Weasley.
"I thought it better not to trouble you with such details."
"If I may continue Molly. Thank you. A baby's thought processes are naturally uncomplicated so it wasn't an overly difficult procedure to extract the fragment of Voldemort from his mind. I think however, judging from what the child know as Mr Potter hears when he is effected by the presence of a Dementor, that we also transferred Harry Potter's memories of that Halloween night. They must have been too intertwined for us to extricate them.
"Voldemort's soul was sealed safely inside the child now known as Mr Potter and he was then delivered to the Dursleys where he could grow up far from the Wizarding World. This link with Voldemort explains the phenomena mentioned by Miss Granger."
There was a moment's silence.