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When Harry, Hermione, Ron, Neville, and Luna had been running around in the Department of Mysteries that day in their fifth year, Luna's fourth, Harry found himself being thrown into a table laden with unknown potions.
They were bottled but smashed upon his contact, the liquids seeping through his clothes and onto his skin.
Feeling no ill effects from them, he continued on, not knowing just how they would affect his future.
When they returned to Hogwarts, Harry refused treatment, not bringing up his run-in with what was likely a bunch of untested potions.
Harry wouldn't find out the side effects for another week when they finally worked their way into his system.
The Butterfly Effect
Harry was asleep, that was something he was somehow sure of.
He was dreaming of a young boy who lived on another planet, a boy who studied such advanced sciences that it made his head spin trying to comprehend what was being taught.
The boy had a best friend, who he loved, and Harry saw their promises to each other, to explore the universe together when they were older.
When he woke up, he could remember the dream so clearly that he almost thought they were his own memories.
The Butterfly Effect
The dreams became more frequent and lasted much longer, and Harry was finally forced to accept that he wasn't dreaming but receiving visions of this alien boy's life.
It felt rather invasive, yet he could not control it.
So instead of trying to force himself to stop dreaming, something which he wouldn't have succeeded in, Harry learnt everything he could.
The Butterfly Effect
They weren't magical but could communicate with their minds, they could control people in a way that was eerily similar to the imperious curse, they could confuse people as though someone had cast a Confundus charm on them.
They could block their minds from the worst attacks, and unlike Legilimency, they could truly read another person's mind.
That was just the tip of the iceberg that was their abilities, hell they could even come back from death!
Harry, who learnt with the boy, found that with enough practise, he too could protect his mind in such a way that not even the most brutal of Legilimency trained witch or wizard could break into.
On the other hand, he really didn't need to know what the other boys in his dorm thought of the girls in the school, or that a lot of the girls and even some of the boys really liked his arse, thank you.
The Butterfly Effect
It took Harry an embarrassingly long time to figure out that the boy he was seeing was actually an alternate version of himself, one born in another universe entirely.
In fact, the other universe was so completely different to his own that there was a show about a person of his species called 'Doctor Who', something which was purely fictional in this universe.
Something about the main character was very familiar to him, and so he thought that he might be someone that his other self knew.
The Butterfly Effect
Over the summer, Harry's way of thinking and his personality began to change, being able to live alongside his other self gave him a new outlook on life as he learnt about things far more advanced than the other humans could ever learn.
There was some kind of time distortion on his visions, for every half an hour he slept, one day passed for his other self, Harry typically slept for eight to ten hours a day.
For every two days, a month passed in this other universe.
By the time the first of September came around, Harry had almost five years' worth of memories from his other-self.
Mentally he was a lot older than his friends, which caused a bit of a disconnection, something they chalked up to the battle at the Ministry the prior year.
Harry let them believe that was true.
The Butterfly Effect
Over the next few months, Harry continued to have these visions, continued mentally maturing at a pace that frightened his friends and some of the teachers.
He'd watched as his other self and his friend renamed themselves, and eventually parted ways in a horrifically upsetting way, which had caused him to wake up in a fit of rage.
Harry had begun to notice that his thought process was becoming very similar to his other self's, and knew for a fact that one day, it would be exactly the same.
He already had a bit of a superiority complex, thanks to his far advanced knowledge.
He'd become far more interested in learning everything he could, frequently sneaking out of the dorm at night under his cloak and reading from the library, he'd even ventured into the restricted section, reading about all the magic they didn't want them to know unless it was explicitly for research purposes.
That was how he found out about Horcruxes.
They were such a horrifying piece of magic, honestly who in their right mind would want to split their own soul? And yet, he found himself fascinated by the disturbing magic.
It felt a bit like reading a good horror book.
And then he figured out that Voldemort had Horcruxes, and that he was one of them.
The Butterfly Effect
He managed to figure out a way to safely remove the soul shard from his scar after three months of intense research.
He'd mixed magic, technology, and a good bit of delving into his own mind to tear it out and store it in the little jar he'd gotten engraved with runes.
Harry poked at the jar with a quill, causing it to screech at him.
"Harry, what the fuck is that?" Ron questioned him as he walked into their shared dorm.
"Some soul shard I found lodged in my scar," he stated calmly.
"...What?"
The Butterfly Effect
After that summer came and went, Harry, Ron, and Hermione were forced to go on what could possibly be one of this universe's worst scavenger hunts.
Over the months their relationship grew strained, especially after Ron's encounter with the locket Horcrux.
Harry slowly became a cruel and rather uncaring person as the days dragged on, a mixture of both the shitty situation they were in and his visions continuing as his alternate self, who now called himself the Master began attempting to take over the universe, he'd even enslaved a planet.
Hermione slowly lost her energy as time passed on, their hunt seemingly fruitless.
The Butterfly Effect
They were walking through the woods in silence, Ron having returned to them the previous day, likely realising his errors.
They'd barely stopped to get a drink when they were attacked by three Death Eaters that had likely gotten lucky with some minor tracking charms.
Harry glared at them and they laughed.
"Aw, is Potter going to cry?"
He gritted his teeth and snapped his head to the side to stare the one that had taunted him in the eyes, he pushed with his mind as he'd seen and felt his other self-do, ignoring the pained gasp that it let out, and spoke in a commanding tone.
"I am the Master, and you WILL obey me."
It didn't stand a chance.
Harry watched in satisfied glee as the Death Eater he'd gained control of summoned knives to itself and launched them at its little friends, both of which went down quickly.
He ignored the horrified gasps from Hermione and Ron as the Death Eater then killed itself.
He smiled and turned to face his two friends, grin slipping slightly as he realised that they were staring at him in fear.
He turned to face the three downed Death Eaters and the reality of the situation crashed down upon him and he sunk to his knees.
He'd done that.
...It wasn't supposed to feel this good, was it?
The Butterfly Effect
Their friendship had been non-existent from that day onwards, with them just barely tolerating each other as they continued their hunt to end Voldemort.
By the time the war was finished, Harry's mentality was completely different from the boy he'd been less than two years prior, and both Ron and Hermione cut all contact with him, something which he didn't mind, surprising everyone else.
Over the course of the next few years, Harry made sure that he only got good publicity, showing the magical population of Britain just how competent he was.
And then he ran for Minister of Magic.
He won.
The Butterfly Effect
Instead of entering a universe where technology was much more advanced than their own universe, the Doctor, Rose, and Mickey ended up in a universe where everything seemed normal.
There were no alien invasions, no government plots to enslave the public, nothing.
And then Rose decided to track down her mother and father.
Jackie Tyler was a witch, she lived in London with Pete who was a wizard and their feline familiar, Patches.
When they visited their house, the two were surprised by them, but not their claims, alternate universes were known to exist to the magical population.
"Just be careful," Jackie told them with a frown.
"Why?" Rose had asked.
"If he finds out you exist, we'll all be in huge trouble, Pete and I don't have a child permit yet, and since you are biologically ours, even if from another universe..." she smiled sadly at them.
The Doctor frowned, "why would you need a permit to have a child?"
Jackie sighed, "it's quite the story."
"I'm sure we have the time."
Minutes later they were all sat in the living room with a cup of tea in hand.
"It started in nineteen-eighty-one," she began.
They listened as she told them of The-Boy-Who-Lived, how he'd been the only one to survive the killing curse, how he'd disappeared, only to come back once he was eleven, constantly being slandered and praised by the public.
She told them how he'd eventually killed Voldemort for good, and how his friends had cut contact with him soon after.
"He spent the next few years charming the public with his confidence, showing us just how he could continue to protect us," she sighed regretfully, "then he ran for Minister of Magic, of course we voted for him."
They sat in silence for a moment as they all absorbed her words, finally, Pete continued.
"He's a tyrant, the first thing he did was have the Wizengamot sign a contract, which they stupidly didn't read, believing it to be in their best interests, because why would our saviour have anything against us?" he gave a disgusted snort.
"The contract binds the entirety of Wizarding Britain to his will," Jackie informed them, "we can't go against him, we'll lose our magic, and be branded as rebels, hunted down and publicly executed."
The Doctor, Rose, and Mickey all stared, horrified, but didn't interrupt.
"He makes us earn the rights to have children, to gain a permit we must do five tasks that prove our loyalty to him, they're horrifying tasks, we just can't make ourselves do them, no matter how much we want one."
They sat in horrified silence for a long while before the Doctor spoke up.
"I promise you we'll stop him."
Jackie laughed bitterly, "don't make promises you can't keep."
The Butterfly Effect
The Doctor, Rose, and Mickey all went different ways as they attempted to stop the man in charge of the magical world.
Rose went to track down some of these rebels she'd heard about, all of which truly had lost their magic, they'd been trying to stop Harry with muggle warfare, something the man was very proficient in, much to their surprise.
Mickey tried to find out anything he could about him online, learning that he owned multiple multi-million companies, and was attempting to ease himself into the muggle government too.
The Doctor found out that there was a ball being hosted in the magical Ministry, and decided to invite himself along, passing himself off as magical thanks to his species abilities.
Wandering around the large elegantly decorated ballroom, he was surprised when a man approached him.
"Hello, sir, would you care for a dance?"
The young man was an average height for a human of his apparent age, reaching to his shoulders, his hair was black and rather messy, as though he'd just gotten out of bed and hadn't yet brushed it.
He had a short beard, neatly trimmed on his chin in a style he believed was called a 'deer tail', and his eyes were a vibrant green.
There wasn't a trace of emotion in those oh so green eyes.
The Doctor, not wanting to be rude agreed, and he found himself spinning across the ballroom with the man.
There was a small smile on the man's face, the only expression he'd seen on his inexpressive face.
And then they were interrupted by a brown-haired man.
"Sorry to interrupt you, sir, but a rebel has been captured inside the Ministry."
The Doctor's stomach dropped as he looked at the man he'd been dancing with as he pulled away, throwing an annoyed look at the other man.
"Which rebel, Neville?"
"Hermione, sir."
The black-haired man growled and stalked to a large throne-like chair, slumping down in it, "bring her in."
A woman about the same age as the man was dragged in by two heavily armed men, chains wrapped around her arms and torso.
The other party-goers stopped their dancing and socialising to watch the spectacle, many people whispering to one another.
Harry Potter stared at the woman in apparent boredom, "Hermione, Hermione, Hermione," he began, "what a surprise."
"This is insanity, Harry!" she cried out, "why do you insist on doing this?" her voice cracked, and she finished her words in a whisper, "what happened to my friend?"
He frowned, "last I checked, it was you who dropped contact with me."
She laughed bitterly, "the boy I dropped contact with was no longer my friend," the Doctor could see her shoulders shaking and realised she was crying, "we were so blind, not seeing that our Harry was dying, and by the time we realised it was too late."
Harry scowled down at her, "people change, Hermione, they grow up," he leant forward in his chair, "those Death Eaters deserved what they got."
"And the rest of the magical world?" she demanded.
"Are perfectly safe," he paused to consider his words, "so long as they follow my laws, something you know very well," he hissed out at her.
She shook her head, letting out a sob, "Harry was such a sweet boy, he never would have done this, you're just a monster that's taken his place."
He looked away from her for a moment, the only sign that her words were upsetting him, it seemed as though they had truly been friends at one point.
"We've been in so many similar situations over the years Hermione, each time I've let you go because, despite everything, I'm still fond of you," he paused to let his words sink in, and then he continued, his words becoming loud and angry, "each time you've come back to make a nuance of yourself, no more!"
She looked to the ground, tears finally falling as she realised that this was the end for her.
"Kill her."
The Butterfly Effect
The Doctor, Rose, and Mickey broke into the man's house the following night and were surprised to see him still awake, feet up on the desk in his office, smoking a cigar.
"You're here to stop me, I suppose," he stated in a relaxed tone.
They stared at him, surprised at how easily he seemed to accept his fate.
"Don't look at me like that, I've always known that someone would stop me eventually, you can't have good without evil, but good always prevails, yadda, yadda, yadda."
He took a long drag of the cigar before he put it out in an ashtray.
"What are you going to do then, lock me up? kill me? strip me of everything I own and dump me somewhere unknown?"
"I-" the Doctor began but couldn't find the words he needed.
"Didn't think that far ahead, did you?"
He shook his head and Harry gestured towards some chairs in front of his desk.
"Sit down and let me tell you a story," he ordered, they warily looked at each other then sat at the offered chairs, the man got up and walked to a cabinet, pulling out a vintage whiskey bottle.
"Whiskey?" he offered them, getting three shakes of the head in return, "oh well, more for me," he stated, grabbing a glass from a shelf next to the cupboard and setting it on the desk, he poured himself a drink and got comfortable, taking a long sip of his drink.
"My story starts in nineteen-ninety-six when fifteen-year-old Harry Potter fell into multiple untested potions."
He told them everything, even bringing up embarrassing moments that the Doctor had been involved with, causing him to become very uncomfortable when Rose and Mickey would turn to gape at him.
He spoke for hours on end about an entire life that wasn't his own, something he'd never been able to talk about to anyone before.
The Doctor stared at the man in horrified fascination.
The Master had managed to steal a body without even knowing, all because a young boy had been forced to live the life of his alternate self.
Harry Potter had never had a chance to be Harry Potter.
The Doctor mourned the person the boy could have been.
The solution to the problem that was his rule over the magical world ended up being rather simple, the Doctor knocked him out when he was least expecting it, and took him to his TARDIS, locking him in a set of rooms set to mimic an apartment, with a bedroom, bathroom, living room, and kitchen.
The door leading to the rest of the TARDIS was removed, and after taking a sample of his hair, he locked him out of the console room, and thus the door leading out of the TARDIS.
He couldn't stay in that universe, and he couldn't be trusted on his own in their universe.
Not to mention just how much of a disaster it would be should he meet his alternate self.
And so, after announcing his 'defeat' to the magical world, and convincing Rose and Mickey that it was safe to be on board the TARDIS and reassuring them that he couldn't get out of his rooms, he checked that the TARDIS was back in working order and returned to their universe.
First order of business; find a therapist that can deal with a megalomaniac with memories that aren't his own adding to his untreated trauma from a magical war.
Simple enough.