[Author's Note: Thanks for reading, and thanks for all the reviews and comments! Some of the suggested solutions to the mystery of Sally-Anne's disappearance were brilliant- I hope those reviewers write their own mystery stories! I would love to read them. Yes, this is the final chapter and the solution to this mystery. For a different take on what happened to Sally-Anne, stand by for my upcoming fic The Book of Abraham the Mage.]

...

"Harry?" whispered Hermione. "Are you all right?"

"I don't know..." Harry heard voices in the distance and hurriedly pulled the invisibility cloak over them both. He could make out the pale oval of Hermione's face under the cloak, even though it was getting dark now. How odd, he thought, that we can still see each other, even though we are both invisible. I wonder if those who are invisible to us are somehow visible to each other as well?

He found Hermione's hand, small and warm, under the cloak and pulled his invisible companion into the bright candy shop. Her breathed against her ear: "Let's get back before the shop closes; we don't want to get stuck out here without access to the tunnel."

Honeydukes was empty at this hour except for a solitary employee, a young wizard in honey colored robes with a glittering golden H on his chest. He blinked in surprise at the slight rustling of candy wrappers as Harry and Hermione tried to walk unnoticed past the overfilled shelves and tables. We are not here, thought Harry, and the young wizard appeared to agree with him, for he merely shrugged and went back to sampling the toffees surreptitiously.

As they wove their back to Hogwarts through the narrow underground stone tunnel, Hermione whispered: "Harry, what was it that happened to you outside the candy shop? You looked as if you had seen a ghost."

"I think I finally realized the truth about Sally-Anne." His voice echoed strangely in the deserted tunnel. Harry paused and looked around. Where were they exactly? It was difficult to tell, since there was nothing to distinguish one part of the ancient stone passage from another. He pulled the Marauders' Map out of his pocket and studied it in the flickering light from his wand. But there were no dots on the map marked "Harry Potter" or "Hermione Granger". Most of the passage they were in lay outside the map; only the entrance to the underground tunnel and the first few feet beyond were visible on the yellowed parchment. We are nowhere, he thought. We are in an indeterminate passage between two knowable locations, lost in the space between Hogsmeade and Hogwarts.

He shook the strange thought out if his mind and turned to Hermione. "Perhaps we should stop here for a moment, Hermione. Maybe we should talk here, before we get back to school. It's difficult to speak privately there."

And perhaps, he thought to himself, it is easier to speak of Sally-Anne here, in this place that is neither here nor there, but somewhere betwixt and between, just like Sally-Anne herself.

Hermione nodded, and they sat down together on the rough stone floor of the passage.

"You know the truth about Sally-Anne?" Hermione's eyes were large and dark in her pale face.

Harry swallowed. "I think I do..."

"Were Sally-Anne and Ariana the same person?"

"Yes. Or no... I suppose that depends on how you look at it..." Harry caught sight of Hermione's exasperated expression and smiled. "Sorry, I didn't mean to be so vague. Yes, Sally-Anne was Dumbledore's sister Ariana, even though Ariana died long ago."

"How... how is that possible?" He could hear that Hermione was trying to keep her voice steady. "The dead can't come back to life, can they?"

"Back to life? No... or perhaps yes... in a manner of speaking..." Harry paused for a moment, struggling to put his sudden strange insight into words. "Ariana Dumbledore returned to Hogwarts, in a fashion, after ninety-five years. But why then? In order to understand how she came to be there, we first need to understand what it was that made her return to the school at that precise point in time. What happened in the early autumn of 1991 that made her come back?"

"You arrived at Hogwarts. Harry Potter, the Boy Who Lived."

"And so did you, Hermione, and Ron, and lots of other people. And so did the two-faced Quirrell, who was hiding a terrible secret under his turban."

Hermione reflected for a minute. "But you and I and Ron arrived at Hogwarts simply because we were wizards who had turned eleven; we came for the same reason children have come to Hogwarts for hundreds of years. There is nothing extraordinary about our arrival, even if there was - and is - something extraordinary about you. But Quirrell..."

"Quirrell had accepted his new post and came to Hogwarts that September because there was something there that his master desperately wanted."

"The sorcerer's stone," whispered Hermione. "Sally-Anne came to Hogwarts shortly after the legendary sorcerer's stone had been brought there. The magical stone that has the ability to produce the elixir of life, to prolong life indefinitely, the stone You-Know-Who wanted so badly... And for a few weeks, before the beginning of term, Dumbledore had the stone in his possession at Hogwarts. But... but Ariana was dead, wasn't she? Surely, not even the sorcerer's stone can awaken the dead, Harry?"

Harry shook his head, slowly. "No, I don't think it can. I think the sorcerer's stone played a role in Ariana's return, but it was not what brought her back."

"I don't understand." How strangely young and vulnerable Hermione looked when she was puzzled!

"Hermione, try to think about what Dumbledore must have been doing in the last few weeks before school began that September," said Harry softly.

Hermione closed her eyes for a moment, thinking back. "Well, he must have been busy getting everything ready for the beginning of term, I suppose. He must have been arranging the schedules for the coming term, talking to the other teachers. And... And he would have been hiding the sorcerer's stone. Of course, that's what he was doing, right before we arrived. And then the Hogwarts teachers would have surrounded the stone with the seven obstacles we faced in our first year: Hagrid would have brought the three-headed Fluffy, Professor Sprout would have provided the Devil's Snare, someone would have enchanted the flying keys... Perhaps Madam Hooch? Yes, it must have been her; there were broomsticks all along the wall of that chamber, and that particular obstacle was a Quidditch challenge of sorts, wasn't it? Professor McGonagall was the one to put the enchanted chess board in place, of course. And then there was the troll. I suppose Quirrell himself brought the troll in." She shuddered a little. "I still dream about that troll at night sometimes. And Professor Snape would have provided the riddle of the potions."

"Funny, isn't it, that Snape should provide a riddle of logic?" Harry muttered as he thought of the sneering potions master. "He doesn't always strike me as terribly cool and rational."

Hermione laughed. "Oh, the riddle of the potions wasn't all that logical Harry. In fact, the riddle was unsolvable."

"What?" Harry blinked in surprise. "But you solved it."

"Of course I did," said Hermione dreamily, "but not by logic alone. You see, Harry, the way the riddle was written, there were two possible answers, not just one."

Two possible answers? Now that's more like Snape! Harry thought to himself. The eternally ambivalent potions master... Even his logic puzzles lack a single definite answer. Her looked curiously at Hermione. "So you just guessed?" Somehow, that didn't strike him as something Hermione would do.

"Of course not. I used logic to narrow the answer down to the two possible solutions, and then I used my knowledge of Snape to determine the correct answer. It was a logical riddle that logic alone could not solve. But in the end I realized that Snape would have preferred the more symmetrical of the two possible arrangements of the potions bottles. Or perhaps you haven't noticed that Professor Snape always arranges potions ingredients symmetrically in the classroom as well?"

Harry shook his head. "No, I never noticed that. But Quirrell made his way past the riddle of the potions as well. How did he do it?"

Hermione smiled. "He must have been reduced to guessing. I don't think he understood Snape well enough to be able to determine which arrangement of the bottles would have appealed to him the most. Unfortunately, he guessed correctly. And then there was the seventh obstacle..."

"The final riddle of them all," whispered Harry. "The Mirror of Erised, the obstacle Quirrell and his dark master could not overcome."

"Put there by Dumbledore." Hermione looked at him, an expression of wonder in her dark eyes. "The Mirror of Erised, the Mirror of Desire. I am beginning to understand, Harry... Perhaps Dumbledore looked into the enchanted mirror of Erised that September and saw his dead sister, the one he had lost so long ago, but never been able to forget."

"But the mirror wasn't in the chamber right away," remembered Harry. "It was somewhere else first, in a sort of storage room, when I found it at Christmastime. Dumbledore must have put it in the underground chamber later. Or perhaps he moved it around... I wonder why. It's almost as if he wanted me to find the mirror. Thinking back, I don't think that it was a coincidence that I stumbled into it that night. I wonder if the sorcerer's stone was already hidden inside the mirror then?"

"I have never seen the mirror, Harry." Hermione's voice was hoarse. "Tell me what it is like."

"The Mirror of Erised?" Harry sighed softly. "It's... It's so lovely that it takes your breath away. When you see it, it feels as if there is nothing else in the room beside that mirror. Or perhaps nothing else in the whole world... It grabs hold of your heart and holds you spellbound. When you look into it, you will see whatever your heart desires, even if you never knew what it was until that moment. The mirror knows you better than you know yourself."

Harry's voice faltered as he recalled the faces of his father and mother in the mirror. "And the strangest thing about the mirror," he continued in a whisper, "is that everything in it appears so real, as if the mirror is a doorway into a different world where everything is possible. And you think to yourself: If only I could figure out how to get to the other side... It is so easy to forget that there is no other side; the mirror is nothing but a glittering surface. It does what all mirrors do: It reflects the person standing in front of it."

His voice trailed off for a moment, and he felt tears stinging in his eyes. Then he whispered: "When I looked into the mirror, I saw my father and mother. They appeared so wondrously real that I thought for a moment that it was possible to bring them back to life. It seemed to me that they were still alive, you see, in that elusive reality hovering right behind the surface of the mirror. If only there was a way to bring them from that reality to this one... "

"I wish there was a way, Harry," said Hermione softly and stroked his cheek. Her touch was gentle, almost imperceptible, but it left Harry feeling strangely breathless.

"What do you think you would see in the mirror, Hermione?" he asked curiously, looking into her brown eyes. "Yourself holding a perfect report card? No, you always get those, even without the mirror."

Hermione laughed. "Perhaps I did desire perfect grades, when I first arrived here at eleven. But now... Perhaps I would see your parents, too, Harry. I wish you could have them back, so you wouldn't be lonely." Harry noticed the sudden flush on her cheeks, and he felt something stir in his heart, something that felt strangely sweet and terribly awkward at the same time. He almost wished that Ron had been there, to alleviate the awkwardness of that moment, and yet he was very happy that he wasn't...

"Do you think that's what happened in our first year, then?" Hermione continued softly. "Do you think Dumbledore looked into the mirror, saw his lost sister, and somehow managed to bring her out of the mirror-? How is that possible? How can you pull a dream out of the mirror and into the real world? How did he turn the memory of Ariana into a girl of flesh and blood?"

"Perhaps the same way I got the sorcerer's stone out of the mirror," said Harry slowly. "Remember that the mirror was enchanted so that only one who wished to find the legendary stone, but not use it for himself, would be able to pull it out of the mirror and into the real world. Perhaps Dumbledore first discovered that enchantment as he stood in front of the mirror, holding the stone he had to hide, and looked at the face of his dead sister. He must have wished desperately to bring her back to Hogwarts. Not for himself, but for her sake... Ariana wanted to go back to Hogwarts after they had pulled her out of school, but she was never able to. Perhaps Dumbledore looked at her pale little face in the mirror and wished in his heart that she would be able to join the new students who were about to arrive at Hogwarts. And perhaps the same enchantment that allowed me to find the stone hidden in the mirror allowed him to find the little girl hidden there as well."

"And then she stepped out of the mirror..." Hermione's voice faltered. "Because he so desperately wanted her to be real, for her sake, rather than for himself. He reached in to the mirror and got his sister back. But I don't suppose he could tell anyone what he had done, so he gave the little girl a different name. He called her "Sally-Anne" because that is who she wanted to be... Yes, that must have been how it happened, Harry. But was she the real Ariana, or just Dumbledore's memory, the girl who came out of the mirror?"

"I don't know. Perhaps she was both..." Harry whispered. He reached out, took Hermione's hand and pulled her to her feet. "Let's go and see Dumbledore, Hermione."

She nodded, and they walked silently together through the dim underground passage until they reached the doorway that led them back into the bright and familiar world of Hogwarts again.

...

They found Dumbledore working at his desk in the warm golden light of a flickering lamp. The rest of his familiar office lay hidden in the deepening evening shadows.

"Ah, come in, young Gryffindors!" Dumbledore put his quill down and beamed at them as they entered. "What a welcome distraction from the tedious, but alas so necessary task of writing the annual report to the Ministry of Magic on the school budget. I am afraid even I cannot perform the sort of magic they seem to be expecting these days. What can I do for you this evening?" He motioned for them to sit, and they found two chairs at the outer edge of the golden circle of light. Something stirred in the shadows behind them, but Harry could not see what it was. Perhaps Fawkes was moving, invisibly, in his dark corner.

"Sir..." Harry looked at the familiar face of the kind old headmaster. He had so many questions to ask, but he didn't know how to begin.

"What's the matter, Harry?" Dumbledore scrutinized him over the edge of his half-moon glasses.

"You... sir..." Harry couldn't get the words out.

He felt relieved when Hermione spoke instead. "We wanted to ask you about your sister Ariana, sir, and about the girl who stepped out of the Mirror of Erised."

"Ah. So you know about that." Dumbledore sat motionless for a moment.

How terribly old and frail he looks all of a sudden, thought Harry. He has that fragile, half-absent look about him that very old people have before they die, when their grip on this world is becoming lighter and lighter with each passing day...

"Sir, are you all right?" Harry touched the headmaster's old, wrinkled hand.

'What-?" Dumbledore looked strangely lost for a moment, but then a shadow of a familiar smile flitted across his face. "I should have known that the two of you would not be able to rest until you knew... It's always so much easier to deceive adults than children. Children always want to know."

"Professor McGonagall knows," said Harry in a low voice, "even if she wouldn't tell us."

"Ah, yes. Professor McGonagall, my partner in crime..." Dumbledore chuckled a little, but there was a trembling note to his laughter. "Poor Minerva! When I confessed to her what I had done and asked for her help in covering up my terrible mistake, she gave me a lecture so severe and furious that the portraits on the wall had to cover their ears. Fortunately, this means that they missed the gist of our conversation. You may not be aware of this, but Professor McGonagall has a rather impressive vocabulary which includes some very colorful Scottish expressions." He smiled ruefully. "But in the end she took pity on me and agreed to help me cover up the evidence of my transgression. She does have a warm heart, you see, and she knew that I acted out of love, however misguided my actions may have been."

"What happened to the girl from the mirror, sir?" asked Harry hesitantly. "Where did she go when she vanished? Was she real?"

"Ah, Harry. Where do vanished persons go?" Dumbledore's voice was barely above a whisper. "Into nothingness, I suppose... Was she real? I'm not sure I know the answer to that, Harry. I suppose she was as real as our memories are..."

There was a faraway expression in his bright blue eyes, as if he was looking at something in the distance they could not see. For the first time, Harry noticed how very similar his eyes were to those of the little girl in the portrait.

"Memories can seem curiously real sometimes," Dumbledore said quietly. "My sister Ariana died ninety years ago. You would have thought that my memory of her would have grown fainter with time. But my recollection of my sister has not faded; it has only grown stronger and more vivid with each passing year. It is the memories we have of the living that begin to blur as we grow older, you see, but our recollections of the dead grow clearer and more luminous over time... My sister Ariana was only here at Hogwarts for a week, but I still recall with perfect clarity how she smiled when she entered the Great Hall arm in arm with her friend Amaryllis, and the way the afternoon sunlight fell golden on her hair as she walked down the front steps of the school. Sometimes I stare at the pale little faces of the first years about to be sorted at the beginning of each new year, and I remember what her face looked like the day she was sorted into Hufflepuff. At times I look at the Hufflepuff students and imagine Ariana walking, invisibly, next to them. And I find myself thinking that that little girl looks pleasant; she would make a good friend for Ariana, and then suddenly I remember that Ariana is dead..."

A tear trickled down his wrinkled face. "But then," he whispered, "a most curious thing happened. I found myself in front of the Mirror of Erised shortly before you both arrived at Hogwarts. There were strange rumors in the air that Lord Voldemort may not be dead after all, and my old friend Nicolas Flamel was terribly anxious about keeping the sorcerer's stone safe. I offered to hide it for him here at Hogwarts, and he readily accepted. I had found the perfect place to conceal the stone: The enchanted Mirror of Erised, in which we see our deepest desires. It was an ingenious hiding place, for desire is the one thing the Dark Lord does not know how to conquer. In fact, it is his own desperate desire for immortality that made him into the dreaded Voldemort. The stone would be safe inside the mirror. And so it was that I found myself in front of the Mirror of Erised two years ago, about to hide the stone in the one place where Voldemort could never find it. But the mirror is powerful indeed, as you have no doubt discovered for yourself, Harry, and I could not resist looking into it for a brief moment. And what I saw took my breath away..."

His voice trembled. "She looked so real, you see, that I began to wonder if I had simply dreamt that she had died all those years ago. She smiled at me and said my name, and I could not help myself: I reached out to touch her, to find out if she was really there. And the the most miraculous thing happened: To my astonishment and delight, my hand found warm flesh inside the mirror. With a beating heart, I took hold of my sister's hand and pulled her out of the mirror, and she stepped out from the silver glass as easily as if she had simply crossed a threshold from one room into another. My sister, here at Hogwarts, a living girl of flesh and blood! I felt such joy that I couldn't care less whether she was real, or whether I had gone mad and was staring at a hallucination. Real or not, she was here, and that was all that mattered. Ariana was bewildered at first; she wondered where she was. But when I told her that she was back at Hogwarts, she laughed with joy. And so I came up with a plan. It was a lunatic's plan, but it seemed to perfectly reasonable and logical to me at the time. Ariana would enroll at Hogwarts; she would be sorted with the new students who were about to arrive at the school. I would bring her to London so she could take the Hogwarts express with the other children, and I would bewitch the Sorting Hat so it would sort her anew... Ariana was so excited about my crazed plan at first, and she laughed when I suggested her new name: Sally-Anne Perks. It was a wonderful new game to her, and she wanted to play along."

Dumbledore sighed heavily. "For a few mad days, I thought it would work... My head was full of wonderful plans for Ariana and for or future together: She would make friends with the little girls at Hogwarts, and I would help her with her classes. She was never terribly bright, but with my help she could surely pass her exams. And later I would find her a job, perhaps here at Hogwarts, so we could always be together..." His voice trailed off, and he sat silently for a moment.

"But it didn't work, did it?" Harry's whisper broke the silence.

Dumbledore shook his head slowly. He turned his face away from the lamplight, so Harry couldn't tell whether he was weeping. "No, Harry, it didn't work. In the beginning, Ariana played along happily, but then a gradual change came over her. She began to wonder... She began to ask would knock on my door at night, when everyone else was asleep, and she would ask me all these terrible questions I did not know how to answer: Who am I? How can I be Ariana if Ariana is dead? Am I real, or am I just a memory, like the portraits on the wall? I tried to reassure her, to cheer her up, but she found no comfort in my answers. She grew more and more distressed and confused, in spite of my efforts to convince her that it didn't matter at all whether she was real; what mattered was that she was here, that we were together. But then one night, she no longer cried. She merely looked at me and said: "I am going back into the mirror now, Albus." I pleaded with her to reconsider, but she had made her mind up. And I understood then that the only thing that could alleviate the misery she felt was the one thing that would break my heart. But I agreed to do it; what else could I do? I loved her, you see, and I could not bear to see her so unhappy. So I led her back to the mirror, kissed her on the forehead one last time and said goodbye. She smiled then, for the first time in several days. And then she stepped back into the mirror and vanished."

"And did you ever... look for her there again? In the mirror?" Hermione's voice was unsteady.

Dumbledore shook his head. "I looked... Oh, yes, I looked, but I didn't see her. I stepped in front of the mirror a few times, but all I saw reflected in the glittering surface was an old man, who had perhaps grown a little wiser." He sighed softly. "At least, I hope so. But sometimes I still wonder what I would do if I ever discovered an even more powerful magic that could recall someone from the dead, a different sort of sorcerer's stone, perhaps. Would I have the wisdom to fling it aside, to toss it into the Forbidden Forest somewhere, or would I be tempted to use it? Even now I do not know the answer to that..."

"Why did you want everyone to forget about Sally-Anne?" whispered Harry. "Why didn't you want her to be remembered?"

Dumbledore looked down. His voice trembled slightly as he responded: "There never was a Sally-Anne, Harry. Sally-Anne was merely a dream who had entered reality for a fleeting moment. She was nothing but a little girl's fantasy and an old man's sorrow. She did not belong in this world, Harry. She had to be forgotten. Professor McGonagall argued with me when I asked for her help in modifying memories and school records in order to extinguish every trace of Sally-Anne's brief existence. She feels that it is necessary to remember our mistakes in order to avoid repeating them. But I thought that she underestimated the foolishness of the human heart: If others were to learn of what I had done, I was worried that they would try to repeat my error, rather than learn from it... In the end, she agreed to help me cover up the traces of my transgression."

"Not all traces, sir," said Hermione quietly. "Nothing can be entirely forgotten."

Dumbledore looked at her thoughtfully. "Perhaps not," he said softly. "But I trust the two of you will understand why you need to keep what you have learned to yourselves."

They both nodded wordlessly.

"But sir?" A sudden thought occurred to Harry. "There is one thing I don't understand."

"Just one?" Dumbledore smiled. "And what is that, Harry?"

"Why didn't you ask Snape to help you erase the memory of Sally-Anne? Why didn't you take him into your confidence as well? I thought you trusted him completely, sir."

"Ah." Dumbledore looked thoughtfully at Harry for a moment. "An excellent question... I do trust Professor Snape, Harry; I would trust him with my very life. And yet... I am not sure that I would trust him with the knowledge that our dreams and sorrows can step out of an enchanted mirror and into reality, if only for a few brief moments."

His glance lingered on Harry face, and then he asked softly: "Can I trust you with that knowledge, Harry?"

"Yes, Professor."

At that moment, a resplendent light illuminated the shadows in the far corner. Fawkes the phoenix had burst into radiant flames of gold and crimson. For a moment he blazed, brilliantly, in the darkened room. But then his fiery splendor begun to fade; the flames grew dim and vanished into darkness. A few moments later, they heard a faint scratching sound, followed by a bewildered little squeak as the newborn phoenix stirred in the ashes.

Dumbledore made some soothing noises at the bird, who fluted softly back.

"Is he all right, sir?" Hermione sounded worried.

Dumbledore smiled. "Of course he is, Miss Granger. Fawkes has experienced this many time before. At least I think he has... I suppose only a philosopher could determine whether I am the owner of one long-lived but combustible bird, or a series of four hundred identical phoenixes who carry with them the recollection of the birds who have gone before..."

His bright blue eyes twinkled behind his spectacles. "I think you had better go now. I have a newborn phoenix to attend to, and I believe the two of you have been sorely missed. I had a visit a few hours ago from your friend Mr. Ronald Weasley and a very agitated house-elf. They were deeply concerned that you had missed both tea and dinner, and that you were nowhere to be found. Between them, they had come up with some fantastic theories to explain your mysterious disappearance. I believe some of their theories involved time travel, vampires, simulacra, and golems, among other things. You really should go and put their minds to rest."

Dumbledore flicked his wand, and the light from the lamp grew brighter. They could see the fledgling phoenix now, a forlorn little creature sitting in a heap of ashes. Dumbledore picked up the little bird and stroked it tenderly.

As Harry and Hermione walked to the door, a light silvery mist wafted towards them from a crack in an ancient cabinet. Harry reached out and touched an evanescent wisp of silver.

"What is this, sir?"

"What?" Dumbledore tore his glance reluctantly away from the little bird on his finger. "Oh, that. Just memories, Harry. Memories of the past... Perhaps I will show them to you some time. There is powerful magic in memories, you see."

...

The End.