This story begins in the Headmaster's Office at the end of the Deathly Hallows, when Harry speaks to Dumbledore's portrait after the Battle.
Chapter 1: In the Headmaster's Office
...Harry raised his hands and all of the portraits quieted respectfully. He sought the one portrait that he wanted to speak to and addressed Dumbledore, who was smiling happily.
"That thing that was in the Snitch..." He started to say. Dumbledore leaned forward eagerly.
"You still do have it? Is it with you, Harry?"
"Well, no." Harry answered, confused. Beside him, Ron and Hermione looked from his face to Dumbledore's painted one, not understanding what was happening. "I dropped it in the forest. I wasn't going to go searching for it..."
Dumbledore stood up in his painted chair.
"Harry, you must find it! You must have all of the Hallows together, they are the only hope of the Wizarding world!"
"But...I don't understand!" Harry stared at Dumbledore with weary eyes. "Voldemort is...dead! It's over!" Dumbledore shook his head sadly.
"It is most unfortunate, but there is still one more great wrong that must be corrected. True, you have defeated Voldemort and ended his reign of terror. This parallel shall go on now undisturbed. But you must go back and fix the first Universe, before the timeline split."
Harry gaped for a moment, and then slowly turned to Ron and Hermione. They both looked as bewildered as he felt. Then a gleam of understanding came over Hermione's face.
"Sir..." she addressed Dumbledore, "when exactly did the timeline split? And are you saying that all of this is not how things were supposed to be?"
Dumbledore beamed at her.
"Right you are Miss Granger! The split occurred on December 31, 1928. But Harry has the power to correct the great wrong because he is the Master of the Hallows!" Harry shook his head, feeling the exhaustion from the recent battle, his grief at losing those who had died, and now his confusion at Dumbledore's words wash over him all at once. He sank to his knees on the dusty carpet and faced Dumbledore.
"Sir...please explain. No more riddles..." His voice was a whisper. Ron clapped a hand on his shoulder and Hermione moved closer to his side. Dumbledore smiled, almost mischievously.
"My dear boy, our story begins as it has ended. With the Riddles." He sat back down again and surveyed Harry over the top of his half moon spectacles. "You do remember what event occurred on December 31, 1928?" Hermione gasped and her hand grasped Harry's other shoulder, hard.
"But that's when he...Voldemort...was born!" Dumbledore nodded at her.
"Yes. His mother died that night, setting into motion the events that have led us to this point. Had she lived, Voldemort would have known love. He would not have become the soulless creature whose body lies below us."
"We already know that...Sir." Ron spoke up for the first time. "What difference does that make? She did die, and he did become Lord Voldemort!"
"In THIS Universe, in THIS parrallel, Mr. Weasley. But the timeline broke apart that night because Merope Gaunt was not supposed to die. A dark wizard, more brutal and deranged than any before him, used a Time Turner to go back to that moment and perform the one act of evil that would virtually guarantee the rise of the Dark Side. He murdered Merope, obliviated the minds of the Muggle witnesses, and left the baby to be raised in that cold, loveless orphanage." Dumbledore paused here and allowed the three of them to digest this information. Hermione closed her eyes in frustration.
"But...who was this wizard? And how did he know that Tom Riddle would become Voldemort? Oh, if he was from the future, I guess he would've known..." She shook her head and looked at Dumbledore wearily. He nodded at her.
"The wizard had been defeated. Time is a curious thing, it has its own rules. The easiest thing for this wizard would have been to go back to his moment of defeat and try to do something different, but Time Turners don't work that way. You can't return to or change a specific moment in your own life, but they do allow you to change an event that may effect your life. As you well know, Ms. Granger." Harry instantly thought of his own adventure with the Time Turner, and how he and Hermione had used one to save Sirious and Buckbeak.
"So, I couldn't have used a Time Turner to stop Sirious from falling through the veil?" He asked, as this was something that would always haunt him. Dumbledore shook his head.
"No. Sirious's death was supposed to happen that way, in this Timeline that has been playing out because of what that wizard did. If Merope had not been murdered, none of this would have come to pass." Harry's exhausted mind was working furiously now, and he jumped to his feet. He felt as though a fire had been ignited inside of him.
"If that event can be undone, then Riddle will never become Voldemort, my parents won't be murdered, Sirious would not be a criminal because there never would have been a dark side..." He gaped at Dumbledore, unable to wrap his mind around what he was saying.
"Exactly." Dumbledore looked satisfied. He clasped his hands together patiently.
"Wait a minute." Hermione said. "We need more information! Who was the wizard? What happened to him after he murdered Merope? How can we go back to that moment if all the Time Turners were smashed? What happens to us if she lives since we are from this Timeline?" She fired these questions at Dumbledore. He only continued to look at her, and Harry thought he saw a look of sadness cross the ancient face. The answer to Hermione's last question came to him moments later.
"It can only be me. I'm the only one who can go back because I have the Deathly Hallows. None of you would exist anymore." He stared at the Elder Wand in his hand.
"Not quite, Harry." Dumbledore said. "You are correct in that this must be a solo mission. As the Master of Death, and Time itself, you are the only one who can go back. Once the Timeline is repaired, this alternate Universe will cease to exist. You will then have the option to merge with your true self, the Harry Potter who was supposed to be, or you can...go on. From all that I have studied about the Hallows, if the mission is successful and you choose to merge with your true self, you will retain the memories you have of this Alternate Universe. No one else will though, because for them, it never did exist." Harry stared at his two best friends. Ron smiled at him, sadly but with courage.
"My brother won't be dead." He said simply. "I won't have to go back there and comfort my mother. I won't have to return to the Burrow and see him there for the rest of my life. George won't have to be alone and in pain."
Hermione stepped forward and pulled Harry into a hug. "Oh Harry. We've been through so much. You need to rest..." Harry faced Dumbledore.
"You haven't answered all of Hermione's questions." Dumbledore looked down and would not meet Harry's eyes.
"It was Grindelwald. He knew that I would be coming for him, and he performed this one final act of evil. It was his revenge, in a way, to cause the rise of the Darkest Wizard ever known. He knew that it would lead to my death, and the deaths of those he despised. But he didn't count on you, Harry." Dumbledore raised his eyes again, and Harry saw that tears were shining in his blue eyes. "You know what happened to Grindelwald. He ended up in his own prison, and was murdered by Voldemort, the evil he himself created. You must go back into the forest and find the Stone. Once the three Hallows are together, you can use their power to go back and stop Grindelwald. At that moment, you will truly understand the power of Love and you will be allowed to make your choice. I know that you need to rest, but find the Stone first. You have done well, Harry. I am so very honored to have known you." Dumbledore smiled one last time at Harry, nodded toward Hermione and Ron, and then walked out of his portrait.