It's Harry Potter's seventh year at Hogwarts. Voldemort was defeated in Harry's fifth year. The wizarding world is at peace.
Albus Dumbledore had told Harry about Horcruxes in his second year after Harry had killed the Basilisk in the Chamber of Secrets and given Dumbledore the destroyed diary. That summer, Dumbledore had taken the young Harry to visit his old friend Horace Slughorn. Dumbledore was seeking as many memories about Tom Riddle that could help them figure out how to defeat Voldemort.
Slughorn was all too happy to reminisce at length to Harry about his mother, Lily Evans. Without knowing it, Harry convinced the older man to give Dumbledore the true memory of Tom Riddle. Harry didn't fully understand why they needed the memory, but he was more than willing to help Dumbledore if it meant keeping Voldemort away.
Horace Slughorn also learned of Severus Snape's treatment towards his students and insisted to Dumbledore he wanted to be the Hogwarts Potions Professor again, on the one condition Snape was sacked. Snape still taught Potions that year.
Neither Harry nor Dumbledore yet knew that Harry himself was a Horcrux, but Dumbledore told Harry that he wondered if Harry shared the Dark Lord's power. He also promised Harry he would look after him, so Harry needn't worry and to owl him if he needed guidance or help.
In Harry's third year, he had barely stepped over the threshold of Hogwarts when he, Ron and Hermione had been whisked away into Dumbledore's office, where Dumbledore told them that Sirius Black was Harry's godfather, and helped Harry to process the news. When Harry and his friends had found Sirius to be innocent, Dumbledore had helped Harry to rescue Buckbeak and Sirius. Dumbledore spent the next two years fighting to prove Sirius' innocence… unbeknownst to the majority of the Order.
The year of the Triwizard tournament, Harry did not go to the Burrow until the day before the Quidditch World Cup was to begin. Instead, he had spent an agonisingly long two days with the Dursley's before Dumbledore picked him up. Together they found the ring, the cup and the fake locket. They chose not to destroy them at this point, but they were kept locked in a cupboard in Dumbledore's office, along with the diary. Dumbledore had given Harry a pile of textbooks and, with a twinkle in his eyes, told Harry to give them to Hermione, for the information inside was sure to come in useful.
At the end of the year, the day after Cedric had been killed, Harry was in Dumbledore's office, discussing Voldemort's return. It was then they theorised Nagini could be a Horcrux, and Dumbledore gave Harry his copy of Tales of Beedle the Bard to read over the summer holidays.
While staying at Grimmauld Place after a mere week in Privet Drive, Harry instantly recognised the locket and kept it to give to Dumbledore once he returned to school. As soon as the locket was on Dumbledore's desk, the headmaster had chosen to destroy the three Horcruxes then and there. He had a feeling they needed to be destroyed sooner rather than later.
That year, Harry had still struggled with his Occlumency lessons, but he had both Snape and Dumbledore to teach him, so he was able to learn the skill at a very basic level. He still had the dreams and learned that his father was a complete prat while he was at school.
After Harry had accidentally seen Snape's none too-flattering-memories of James Potter, Dumbledore had taken Harry off school grounds – much to Umbridge's outrage - to see Sirius and Lupin at Grimmauld Place. The pair had assured Harry that James hadn't been all bad; they never denied James had been a prat, but they insisted he did mature and grow out of it. Dumbledore hadn't joined in their conversation, instead choosing to read Muggle magazines at the other end of the table and have a conversation with Kreacher about how Sirius treated the house elf.
McGonagall too had found out what Harry had seen and made a point to talk to Harry about James Potter, Lily Evans, their friends and time at Hogwarts.
Dumbledore was upfront and honest with Harry that year and told him of his suspicions of the connection between Harry and Voldemort and even explained why he was being careful around Harry that year. Harry saw less of the Headmaster than he would've liked, but this motivated him to work harder at his Occlumency lessons so that he and the Headmaster could speak without fear once again.
It was just after Arthur Weasley was attacked that Hermione had her epiphany.
"The Basilisk!" She had squealed suddenly. Every member of the Weasley family, and Harry, had been around Arthur's bed and turned to look at her in surprise and confusion. Hermione had violently grabbed Ron and Harry's wrists and dragged them out of the room while whispering to them at top speed. "Harry, those textbooks Dumbledore gave to me through you, they said Basilisk venom can destroy Horcruxes! There should still be fangs lying around from the Basilisk in the Chamber of Secrets!"
Harry had confided in select members of the DA about Horcruxes and asked for their help to figure out what and where the remaining Horcruxes could be. Luna had been the one to suggest the diadem and told Harry who the Ravenclaw ghost was.
The diadem had been found and destroyed two days later.
Sirius did not die in the Department of Mysteries. Instead, it had been Bellatrix Lestrange to fall through the curtain after another Death Eater had fallen to a leg locking curse and pushed her. The Order had defeated the Death Eaters that night.
As everyone had been leaving the Ministry, Voldemort had appeared in front of them in a blinding rage. Lupin was hit by Crucio while Sirius was hit by Imperio. Voldemort had Sirius tie up the Order and keep them out of the way while he duelled Dumbledore.
Nagini had been slithering around on the floor by Sirius' feet and Harry, Ron and Hermione, who had each developed the habit of always having a Basilisk fang on their person, each stabbed Nagini with the fangs.
Voldemort had screamed - an inhuman sound that would haunt the nightmares of everyone who heard it - and sent a killing curse past Dumbledore, towards the three teenagers who had still been standing over the dead snake's body. Without thinking, Harry had leapt in front of his friends to have the curse hit him squarely in the chest.
Harry Potter had died.
But he had also come back.
Tom Riddle stood before the group as nothing more than an ordinary man once again. Sirius had broken the Imperious curse he had been under, Lupin was released from the Cruciatus curse and Harry had faced Riddle with Dumbledore at his side and his friends behind him.
Riddle tried to kill Harry for the third time in Harry's life. But Harry's Expelliarmus had broken through.
Tom Riddle's body had hit the group with a sickening, final thud as the Minister for Magic had stepped onto Ministry grounds.
Every member of the Order had been pardoned, offered jobs within the Ministry – including Sirius and Lupin – and were regarded as heroes.
Harry, Ron and Hermione were just thankful they would be able to continue their studies in peace, without having to worry about plots or schemes or saving the world.
Their sixth year passed without drama, save for the almost weekly duel Harry found himself in with Malfoy.
Finally, the trio had made it to in their seventh and final year at Hogwarts.