Disclaimer: Being neither British nor Japanese, it should therefore come as no surprise that I own neither Harry Potter nor Naruto, nor anything from their respective franchises.

A/N: The idea for this fic came from pondering the question: "Could any Naruto character be dropped into the Harry Potter world and have a positive effect without being ridiculously overpowered?" If you're looking for Super!Harry, you won't find him here. I don't want a prohibitive wall of A/N here so I'm putting more information in a longer A/N at the bottom.

This fic will contain spoilers for most of the Harry Potter series, as well as bits of Naruto.

For those wishing to skip ahead (and possibly find themselves confused), Book 1 begins with Chapter 11 (or 1-1).

Text inside «» is in Japanese.

Book 0: Umino Iruka and the Wizarding World

Jutsu mishaps are a fact of life in any Hidden Village. While unlikely among experienced shinobi, those with less experience are more accident-prone. Gather several dozen adolescent students in one place and the likelihood rises to near certainty. Backlashes, chakra dysfunctions, misfires, and (unintended) explosions are all unpleasantly familiar to anyone that has spent time teaching at a shinobi academy; an accepted hazard of their chosen profession, so to speak. Usually such mishaps follow consistent patterns: Students trying to master the same jutsu will usually make similar mistakes. Thus, their teachers are generally well-prepared to handle the aftermath.

Real trouble comes from the more unusual failures, especially those that come from ambitious students attempting jutsu well beyond their current abilities, or even trying something completely at random. These attempts can produce truly unpredictable results. When the attempt (and mishap) in question comes at the hands of the son of history's most unpredictable shinobi, a man well-known for frequently doing things previously believed to be 'impossible'...

The screams weren't all that surprising; bad mishaps could get messy, and injured and/or frightened children are prone to screaming. Even in his disoriented state, Iruka snapped into action, pulling supplies from the large first-aid kit he was carrying in case of training mishaps (like this one) and moving to the obvious casualty in front of him. Even without knowing medical jutsu, he'd taken the advanced classes instituted by Tsunade-sama to make sure even non-medics could stabilize their wounded comrades until more extensive aid could be reached.

The teacher began assessing his patient's injuries: burns, shrapnel wounds, blunt trauma, concussive injury, chakra depletion - wait, no... The chakra levels were low, but none of the usual signs accompanying chakra depletion were present. How did a civilian manage to get hit in a training mishap? It was only then that Iruka's mind finally registered his surroundings. Instead of being outdoors in a training field adjoining the Academy, he was in an unfamiliar room whose construction and decoration was of a style he'd never seen before (though the scorch marks, blood spatter, and other damage were probably a recent addition). Far from being a ten-year-old aspiring shinobi, the casualty was a civilian woman, late 20s or early 30s, with pale skin and long silver-blond hair, reminiscent of a Yamanaka. The only other person present was a girl of similar age to the students Iruka had been working with, though she too was unfamiliar, whose features suggested a close relation to the woman. It was from this girl, now standing in clear shock and confusion, that the screams had issued.

Pushing aside the obvious questions about where in the Sage's name he was and how he'd gotten there (though the answer to the latter likely included the name 'Uzumaki'), Iruka frantically began bandaging the woman's shrapnel wounds to try and staunch the bleeding. The number and severity of the wounds, though, was proving problematic. He needed more hands, but in his current condition his chakra reserves weren't nearly full enough for a Shadow Clone. Only one option, then: «I know you're scared right now,» he said to the girl in his best reassuring 'teacher's voice', «but I need your help. I need you to press this gauze pad here to help stop the bleeding; can you do that for me?»

The girl's only response was a baffled stare, followed by words Iruka couldn't understand, though he assumed they meant something like 'what are you saying?'. A language barrier. Just what he needed. Pushing down his frustration and worry, he kept his face, tone and body language as calm and reassuring as he could, adding gestures to his instructions as he again asked for her help. Soon, even despite their trouble communicating, the two were working together to help the woman he could only assume was the girl's mother.

As they worked, Iruka's mind idly examined the pattern of the damage in the room, the woman's injuries, her positioning on his arrival, and the girl's position and condition. It was clear that there'd been some kind of small explosion, a fraction of the power of a standard explosive tag, but made just as dangerous by the woman's proximity and the generation of shrapnel. Also apparent was that there had been at least a moment's warning, and that the woman had moved to shield her daughter, likely worsening her own injuries as a result. This assessment only strengthened Iruka's resolve to make sure the woman survived.

Finally, after several fraught minutes filled with bandaging, pressing, checking, and the occasional suture, Iruka and the girl were able to sit back and catch their breath, their patient reasonably stable for at least the present. After sitting and staring at her mother in shock for several moments, the girl started, and dashed to a wrought-iron spiral staircase at the center of the circular room. Iruka hurriedly followed down the stairs into a large circular eat-in kitchen, to find her standing before a hearth of rather impressive size in the neighboring room. To Iruka's confusion, she reached into a vase on an end table nearby, throwing a handful of some sort of powder into the hearth, which promptly lit with green fire! Confusion turned to alarm as the girl moved not away from the strange blaze, but towards it. Iruka dashed over and grabbed her before she could burn herself, causing the girl to shriek in alarm and try fruitlessly to pull away.

Upon realizing who held her, the girl immediately began speaking rapid-fire in her language, and gesturing towards the now-dying green flames. Surprisingly, her tone wasn't that of panicked babble, but of confident urgency and command. Curious, Iruka reached tentatively towards the diminishing flames and was surprised to feel that no heat radiated from them. Reassured, if still confused, he released the girl, allowing her to throw another handful of the unassuming powder into the hearth, causing the green flames to flare back up.

To his dying day, Iruka would never admit to the words he used in front of a child when he saw said child shout something then stick her head into the green fire. While she bent before the flames, her head and neck seemed to blur and fade, as if they weren't entirely there, though she showed no ill effects from this, nor any sign of burning. After a few moments of muffled conversation with an adult male voice, the girl stood and stepped back, gently urging Iruka away from the hearth at the same time. The reason for this was soon made clear, as the green flames flared higher than ever as the most bizarrely-dressed man he had ever seen (and between Naruto and the YOUTHful lineage that was saying something) seemed to appear spinning within the flames before stepping out as they died out behind him.

The new arrival was tall and lanky, with stringy blond hair and garments of an unfamiliar cut in a truly eye-watering medley of colors. Sparing a brief glance towards Iruka (obviously to assess whether he posed a threat), the man turned to the girl and spoke gently but urgently to her. She replied and pointed to the stairs, turning the man's worried expression to one of alarm and concern. He rushed to upstairs to the woman's side, pulling a polished wooden stick from his clothes (tool? weapon?) and proceeded to wave it in precise patterns over the woman, while muttering words that sounded like yet another unfamiliar language. To Iruka's surprise, this resulted in a number of colored lights, both from the stick to the woman's body, and appearing around her with color variations around her injuries - whatever that stick was, it was clearly a tool for these diagnostic jutsu, with the movements presumably taking the place of hand seals.

Before Iruka could further ponder what he was seeing, the man rushed back to the hearth, throwing in more of the green powder, before doing as the girl (his daughter perhaps?) had and sticking his head into the flames, shouting urgently to whomever he was trying to contact, before withdrawing. Soon the flames flared up again and emitted three new arrivals, all of whom were dressed in similar pale-green uniforms and wore the familiar confident focus of medics responding to an emergency. Within moments, they too were waving sticks over the woman, pouring phials of medicine into her mouth, and examining Iruka and his young helper's work. Throughout this, the girl clung to her father, who wrapped his arms around her as both tensely watched the medics and their patient. Iruka simply stood and observed, ready to help anywhere he could and uncertain of what else to do in the strange environment.

Once the initial frenzy of activity calmed, Iruka relaxed and one of the three medics (a petite woman with an athletic build and short-cropped sandy hair) stood and approached Iruka and the father-daughter pair. In a gentle voice, she asked several questions, most of which were answered by the father; his daughter's surge of confidence had receded now that adults had taken command, and she gave only short answers in a tone of distracted timidity. At her prompting, the father and daughter released each other and stepped apart, allowing the medic to perform another diagnostic jutsu, this time on the girl, the results of which clearly satisfied the medic. She handed the girl another medicine phial, which was shyly taken and soon consumed. It was apparently a sedative of some type, as the girl relaxed within seconds before her eyelids began to droop and she rapidly fell into an emotionally-exhausted sleep as her father scooped her up into his arms. This seemed to be the cue for the medic to turn her questioning to Iruka, who tensed slightly as the strange stick was suddenly pointed near him (but not at him, he noticed, similar to a weapon held ready but not aimed). «I'm sorry,» he replied, «but I'm afraid I don't speak your language, and I suspect you can't understand mine.»

«Fortunately, even if she can't, I can.» The father had laid his daughter on a couch and covered her with a light blanket, and was now sitting in an adjacent armchair. After a brief bit of the local language directed at the medic, he continued, «I've traveled rather extensively, and one of my longer expeditions had me living in Kyoto for several months. Relying on a phrasebook or retaining an interpreter for that long would have been less than practical, so I did my best to pick up Japanese.»

Iruka could only assume that 'Japanese' was the local term for the language of the Elemental Nations.

«Travelogues aside,» the man continued while gazing both at his wife and at something far in the distance beyond her, «Healer Haff said that were it not for your efforts, my wife likely would not have survived, and so on behalf of all of my family I offer my most sincere thanks. Incidentally, she wants your permission to perform a diagnostic scan to check you for any injuries or other damage.» Iruka nodded to the 'Healer', who performed another set of diagnostics before pulling out another phial of medicine and holding it out and speaking further. «She says you show signs of a mild concussion, which is what the potion is for, and a number of minor bruises and moderate fatigue. Her prescription is to drink the potion, rest, and let the bruises heal naturally, taking no further potions for at least three days to avoid unpleasant interactions. My advice would be to drink your potion and begin your resting by having a seat,» the man finished dryly, indicating another armchair.

Iruka sat as he was bid, glad for the rest after such chaos, the beginning of which he was going to need some quiet time to piece together at some point. He looked at the 'potion', seeing it was viscous and pale blue, contained in a handmade glass phial with a cork stopper - a somewhat archaic container, but given the use of terms like 'potion' and 'magic', these people might not have access to more modern technologies and jutsu knowledge, even if their local form of jutsu were clearly well-developed on their own. The important question was, could he trust this phial of an unknown substance, given by a strange foreigner using strange jutsu in a strange place? While both of the adults looking at him were clearly suspicious of him, he hadn't exactly arrived in the best of circumstances, so a lack of suspicion would have been more worrying. Regardless, the whole situation made no logical sense as anything but a surprise to these people: Iruka could think of no plausible scenario that would paint those presently around him as default hostiles. If they wanted to poison or drug him, there had been and would be far safer and easier opportunities to do so. Thinking this, he uncorked the phial and, while the contents smelled unpleasant, they didn't smell like any poison he knew, so he shrugged and drank his medicine.

The 'potion' was certainly not pleasantly-flavored, worse than field rations but at least better than Sakura's custom-made pills. Some of the grogginess Iruka had been feeling since the incident began to clear almost immediately, making him wonder about whether these medicines could be brought back to Konoha, as that kind of rapid curative could be potentially life-saving on the battlefield. Glancing back at his host and the medic, he could see the former scrutinizing him intently, while the latter just looked exasperated and somewhat bemused.

«Now that that's out of the way,» the father began, «Healer Haff's other question is one that I share: What can you tell us about what happened here? I would also add a related one of my own.» A civilian might have missed the man surreptitiously bringing his stick into a ready position. «Why were you in my house?»

A/N: Yes, Iruka just saved Pandora Lovegood. Luna is by far my favorite Harry Potter character, and since I wanted Iruka to arrive a while before Harry starts Hogwarts anyway, Pandora's accident seemed like a good place to start.

So, about this fic: This is going to be a long one, folks, though you may have guessed that from the "Book 0" at the top. My current plan is to update on a fortnightly basis, though I'm putting up two more chapters as part of the initial posting. If that update schedule changes, I'll note it in the chapter posted before the change and edit this A/N as well. On that heading, it will likely be at least a couple of years before I get through everything I have planned, though I intend to step up my release schedule significantly once I've finished the final chapter. For now, I'm keeping one year's worth of buffer to start with, in case of incapacity, writer's block, or Nargles.

I intend this to be effectively a single-point-of-divergence AU. That said, a few things may be slightly different from how JKR portrayed them; this will generally be me doing my best to make sure everything makes sense. An unfortunate side-effect of transitioning a story from children's lit through to something more grown-up is that elements that were originally fun, whimsical silliness turn into plot holes. A not inconsiderable amount of my effort in writing this story has been filling those holes without diverging unnecessarily from canon.

Iruka will not be a combat powerhouse in this story. He'll be easily capable of dealing with an average witch or wizard, but the major part of his influence will be through advice and guidance. Harry (and a number of other characters) will have a more involved mentor and advocate, and the Order will eventually benefit from the experience and knowledge of a veteran professional soldier.

I'll try to avoid wall-of-text A/N in future chapters, but will occasionally use them to explain important things that I just can't find a place to handle in-story, and to answer questions that seem like they'd be of general interest.

Many thanks to Akatsuki210 for putting up with me and for beta-reading my work. Also, many thanks to robst and MaiKusakabe for their stories "Harry Crow" and "The Colours of the World", respectively: The former inspired me to try and make everything make good sense (and is also easily the best HP fic I've ever read) and the latter helped inspire some of the structure of this story. Quatermass and his/her many fun crossover stories also provided inspiration. Further thanks go to Sinyk for inspiring my ASCII-Art scene-break dividers, and to White Squirrel for inspiring me to have some form of a table of contents. And, of course, thanks to all of the above and others for writing amazing stories that have provided me with countless hours of enjoyment.

Thanks for reading, and remember that your reviews can improve the quality of the story! Also, I'm mostly writing this because if I don't, nobody will, so if you like this idea (which is actually my response to a prompt I posted on DZ2's forum) please feel free to write up your own version! Just let me know if you do, so I can read it, right?

My first fic recommendation is "Harry Crow" by robst - a novel-length AU that does an amazing job of characterization and story.

Posted 21 May 2017

Updated (A/N only) 30 July 2017: Added fic recommendation

Updated (A/N only) 15 September 2017: Added ToC and updated acknowledgements.