Disclaimer: Neither "ElizabethRobinThales" nor her orthonym accept responsibility for any Everett Branches created by your decision to read this work of fanfiction.
Author's Notes: I'm not sure how commonly Mary Poppins' origin in mysticism is known. As far as I can tell, no other fanfic on either FF dot net or AO3 utilizes this aspect of the Mary Poppins stories in any capacity, so I think it would be prudent to clarify some things so you can have a better idea of what to expect going forward.
P.L. Travers held a set of beliefs that would fall under the general umbrella of "Western esotericism." Her writing was markedly influenced by the occult and by Blavatskian Theosophy (which in turn was inspired by Hinduism and Buddhism and Neo-Platonism). She sprinkled a bit of Jungian nonsense in there as well.
This is not a romance or a familial drama or a variation on the common theme of "a concerned adult rescues Harry Potter from a neglectful upbringing." Mary Poppins' young ward in this story may happen to be the main character of HPMOR, but this is first and foremost a Mary Poppins story in the spirit of the original books (with some structure and arrangement inspired by the movies - the books may be rich with Eastern symbolism, but they're relatively poor in plot, amounting to little more than a series of barely connected vignettes).
Also, this fic probably isn't an infohazard. Just throwin' that out there for no reason, don't worry about it.
THE END OF ALL THINGS
Chapter 1 - Harry's Oxford
If you're looking for Number Eleven, Paper Street - a small, dingy-looking old house whose occupants mostly use it to keep the rain off their books - you won't soon find it if you sally forth at once towards the neighborhoods east of Florence Park in hopes that you'll eventually stumble across it while wandering aimlessly through the streets.
No... Your best bet is to make a beeline for the local Recreation Ground.
On the outskirts of the park in an open green, far from the slides and swings and other such trappings of childhood sport, you should see a lone oak tree - its red leaves mottled with splotches of yellow and brown - with a school satchel leaning against its trunk and a young boy sitting on its lowest-hanging branch reading a book considerably thicker than what might be expected for his age.
This, conveniently enough, is Number Eleven's youngest resident, Harry James Potter-Evans-Verres - bibliophile, self-described 'aspiring rationalist', and somehow connected with the summoning of a total of three fire engines.
Harry isn't legally allowed to be in public right now. His father told him as much before leaving for work.
"Where do you think you're going, young man?"
Harry's hand had already been grasping the doorknob. "Well I haven't been out of school for even a day yet so my brain is still running on a school schedule and I'd normally already be walking to school by now so I thought I'd take some nonfiction books to the park and read them there instead of here so I won't be tempted by all our other books or the television."
Number Eleven is less than ten minutes' walk from St. Menas Primary, and the park is midway between them. Harry's parents usually didn't arrive home from work until nearly dusk, so he had developed a habit of stopping at the park after school, ostensibly to do his homework.
The weekly vocabulary was a joke, and the daily maths never took longer than ninety seconds to complete (it was intended for seven-year-olds, after all, and most seven-year-olds don't know what a logarithm is). But once he'd gotten the government-mandated twaddle out of the way, Harry would spend the rest of the afternoon reading while sitting on what he was starting to think of as his branch (though we might note that the branch does not, in fact, belong to Harry, it belongs to the tree).
His father - an Oxford biochemistry professor named Michael Verres-Evans - sighed. "We haven't pulled you out of the school system yet, Harry. If you're seen in public during school hours, I'll get a fine."
The day before, Harry had been given a three-day fixed-period exclusion for biting his third-year maths teacher (she didn't know what a logarithm was). He stayed in the park for an extra hour that evening, sullenly resenting the rest of the world for having the nerve to be so bloody stupid, and anxiously brooding over the possibility of losing his television privileges when his parents found out what he'd done.
While he did lose his television privileges for a week, his parents had reacted less irrationally than he anticipated. They actually agreed with his assessment that it would be in the best interests of all parties involved if he were privately tutored.
"How much will they fine you if I'm caught outside?"
"Sixty pounds sterling."
Harry's father then expressed the sentiment that he was probably making a terrible mistake and shouldn't ought to be leaving Harry alone for even a single day, and impressed upon Harry that he would be quite put out should it come to light that Harry had so much as opened the front door.
"I'm going to spend every free moment between classes phoning around for a reputable childcare agency. If things pan out, I'll have found someone to watch you by tomorrow, so how about if we postpone the television embargo until there's an adult around to enforce it?"
Harry looked back at the door, and noticed that his hand hadn't yet released its hold on the doorknob.
His father sighed again (Harry did notice that adults would sigh more often while talking to him than while talking to other children, but he couldn't seem to figure out why).
"It's unseasonably cold out there, son. Just stay inside and watch the telly. If you don't, I'm going to give it away and we won't get a replacement until Christmas. Don't do that to your poor mother, you know how she likes her evening programs."
Harry let go of the doorknob.
"Please try to have a bit of self-discipline today, Harry. Don't set anything on fire or get into any other sort of mischief but especially don't set anything on fire."
Professor Verres-Evans knew his son well.
Not one hour after Harry's mother - Petunia Evans-Verres - left for work at the local Barclays, Harry found himself staring out the window. No one's out and about, he thought. Most people are probably either at school or at work right now. And it's less than a five minute walk. No one would see me...
And on a split-second decision Harry jumped up, threw on a light jacket, and began stuffing books into his satchel.
The wind abruptly intensified and the branches over Harry's head began to sway dramatically. Reading in these conditions was going to be a chore. Harry would have suspected a storm, but it was a bright and cloudless day.
Maybe I should just go home, he thought. I didn't think to bring any snacks and it's about time for -
A particularly strong gust snatched the book right out of Harry's hands and carried it off just as a deep, snapping crack came from higher in the tree.
Yep, it's time to go.
Harry swung himself down to the ground, but the wind died down and stagnated the very moment his feet touched the grass.
Typical, he thought.
Stephen King's The Gunslinger - the book Harry had been reading before the wind so thoughtlessly blew it away - was sitting several metres from the base of the tree in the middle of the green. Harry retrieved it, then paused and looked up at the sky. The world at large seemed not to have taken any notice of the anomalous burst of blusteriness. It would be easy to just climb back into the tree and forget that anything had happened.
Well, I suppose it's for the best that it happened, anyway. Mum very well might sacrifice her lunch break to come home and... make sure that I'm not...
The air was completely still, yet the leaves were still rustling in a most peculiar manner. Harry glanced back out of curiosity and, to his utter disbelief, a woman was standing on the top of the tree as though the leaves formed a solid surface.
Then Harry blinked again, but the woman was still atop the tree.
"H-hey!" he yelled. "How on earth did you get up there?"
The woman looked down at him sharply. Her expression was angry, outraged, and astonished all at once. "I'll thank you to mind your own business, Mr. Potter-Evans-Verres!" She didn't shout, but her voice carried regardless. "As it were, I have always been here."
"How do you know my - But there wasn't anybody in the - It isn't possible to have always -"
"Everything is possible. Even the impossible."
Harry fixed her with a look. "Ma'am? That is absurd."
"What an impertinent thing to say!" The woman sniffed. "Haven't your parents taught you to respect your elders?"
"I respect the truth." Harry's gaze held steady. He had encountered adults like this before (or so he thought).
"I see. Is that why you bit Mrs. Aldwinkle? She lied to you about not knowing what a logarithm is?"
Harry was taken aback. "How could you possibly know about that? Who the bloody heck are you?"
The wind again gained in strength. The branches of the tree bent wildly, creaking and groaning in protest, but the woman calmly began to walk - not climb, but walk - down the branches as though they formed a spiral staircase. Every step she took just so happened to land on a branch that had been blown out of place by the wind, and the attachment points of several branches perceptibly repositioned themselves higher up or lower down along the trunk to more readily accommodate the strange woman's walking.
Oh my god she's magic, there's no such thing as magic but apparently that doesn't matter.
She was closer to the ground now, and the branches were helpfully moving themselves out of the way of Harry's view. Her hair was shiny and black. She wore a long blue coat with silver buttons and a blue hat to match, and she was holding a carpet bag and an umbrella with a handle shaped like a parrot's head.
No... She couldn't be...
The final step in the emergent spiral staircase was Harry's own branch (which, despite being the lowest-hanging branch and seeming to be at least a foot closer to the ground than he remembered, was still more than two feet higher off the ground than Harry was tall). And then somehow, impossibly, the woman continued walking down as though she hadn't noticed that she ought to have fallen.
Harry could no longer contain himself. "You're Mary Poppins!" he shouted, just as the woman alighted primly upon the ground.
"I beg your pardon?"
"It's the wind," said Harry. "Mary Poppins always comes down out of the sky with the wind - just like you've just done - and you're dressed like her, and everything about what just happened blatantly violated the laws of physics, and -"
The woman harrumphed. "Well you already know the name, presumably by means of some sort of narrative depiction -"
"It's a series of books, and they made a movie adaption, though you don't look like Julie Andrews -"
"Of course I don't look like Julie Andrews! Julie Andrews is Julie Andrews and I am myself!"
That wasn't an outright denial. Any sane, rational person who was not Mary Poppins would certainly scoff at being directly accused of being Mary Poppins.
You observed the impossible, Harry thought to himself. Your first impulse was to label it 'magic'. You have to accept your observations as fact. It would still be true even if she DID deny it, she's DEFINITELY Mary Poppins.
Harry decided to carry on as though it were a given that the woman was, in fact, Mary Poppins. "You're quite a well known fictional character, you know. I don't know what business you think you have showing up here in the real world but -"
"Every world is a real world. It's quite arrogant to presume that any particular one of them is more 'real' than the rest for something as trivial as simply adhering more tightly to the laws of probability and causality than average. And if that were the basis for determining so-called 'realness', then by virtue of your having observed my arrival, this world couldn't be as 'real' as you believe it to be, now, could it?"
Harry's eyes widened.
"Now then... Young as you must have been when you saw that movie, surely you watched it with your parents - Or perhaps the movie is decades old by now and would have been widely known when your parents were younger?"
Harry's eyes narrowed. "Why does that matter?"
"It doesn't, but that's how these situations tend to pan out." She turned and took a step, then paused and looked back. "I never introduced myself, did I? How dreadfully rude of me." She smiled, and it didn't quite reach her eyes. "I am Helena Vela Boot."
"No you're not, you're -"
"Come along now, you need to get home before your nosy neighbor phones the authorities. Don't dawdle, spit spot!"