The Butterfly Effect. A part of Chaos Theory that, summarized, states that in a small change in a system, can cause massive changes because of it. The typical example is the flapping of a butterfly's wings on one side of the world displacing air in just the right manner that it causes a sort of dominoes effect that eventually leads to a hurricane clear across the globe. Obviously, the hurricane wouldn't be the only effect, but to us, as humans, it is a dramatic enough change that it drives the point home that even the smallest of actions can have unforeseen and far-reaching consequences that we might never expect.

That is a small example, the flapping of a butterfly's wing. In a theoretical sense, a bigger change might cause even larger consequences. That isn't necessarily true, though. The size of the change need not be directly proportional to the results later on down the road. For example, what if the Y-chromosomes of two future heroes had instead been X-chromosomes? Shall we explore just how far reaching such a simple set of changes might go?

Izuku Midoyira, age nine, had gone into the city with his parents on a small shopping trip to get a birthday gift for his best friend, Katsuko Bakugou. Ka-chan was... Difficult, to say the least for the young, hero-obsessed boy. On one hand, he wouldn't trade her for any other friend in the world.

On the other, she was quite a lot to handle. At the age of nine herself, she was significantly taller than him, and had a powerful quirk that means she's almost guaranteed to one day be a great hero. At least, if her attitude wasn't so rotten. She was violent, loud, and rude to most everyone, save the Midoyiras. Izuku, she protected from the bullies that would otherwise make fun of his quirkless nature, and she had always treated his parents more like her aunt and uncle, as opposed to just the parents of her friend.

None of that mattered to Izuku, as to him Ka-chan could effectively do no wrong. That isn't to say he couldn't see her flaws, numerous as they were; he just shrugged them off as, 'It's Ka-chan,' and left it at that. The one thing he did get a bit angry at her for, though? Her insistence that he can't be a hero, since she can't always be there to save him. That did nothing to deter the boy, though, instead steeling his young resolve, making him more driven than ever to prove her, and the world, wrong. He'd even sworn to her that one day, that he'd take the U.A. entrance exam and show her that he can be a hero, just like her.

Today, though, was a simpler time for the boy, his mom held his hand as they walk through Tokyo, trying to think of what the explosive girl might like.

The solution came surprisingly easily.

Dragging Inko over to the window of a shop, he started bouncing on the tips of his toes in excitement. "Mama, Mama, look! Ka-chan's been complaining about her hair getting in the way, maybe she'd like that?" He asked, pointing to a set of scrunchies and hair ties in the iconic form of their favorite hero, All Might.

Looking down at her son, she nodded, letting him drag her into the hero-novelty shop. "Slow down Izuku, they're not going to run out of them!" she teased him as they looked around the store, trying to find the hair-ties.

After a much more enthusiastic search than one might expect of a nine-year old looking for such a thing, the family approached the counter, paying for their purchase. "Okay, can you think of anything else Izuku?" Hisashi asks his son as they start to leave, the father having decided to throw the lad on his shoulders. After all, he wouldn't be able to man-handle his son forever.

"Daaaad, let me down! I'm too big for this!" Izuku half complained, his laughter giving his pleasure away at the 'childish' activity.

Inko was about to say something to her boys, but it was cut off by a loud crack, accompanied with a deafening boom. Turning towards the noise, she almost asked what's going on, only to be stopped as a wave of force and displaced air careened down the street, hitting the family and all of the other shoppers. Hisashi fell backwards, his back slamming into a car as Izuku rolled across the hood of the car. If one were to look closely, they'd be able to see the stream of blood flowing from the back of his head as he slumped down the side of the vehicle.

"Izu!" Inko yelled as she dove for her boy, sub-consciously using her quirk to pull his clothes towards her in a vain attempt to keep him from flying further away. She hadn't even noticed her husband's state yet; the man, having been a professional hero for a few years, she assumed he'd be able to take care of himself. While her mind could never hope to be torn from the safety of her baby, more pressing matters reared their ugly head as the shockwave now had a cloud of debris and destruction following it up. Loose garbage, uprooted signs and even individuals were all caught up in the maelstrom as it reached her, forcing her to hit the car with her husband as a sign hit her head, knocking her unconscious from the blow. Izuku, having landed between the two cars when his tumble had ended, was largely unscathed save for a few bumps and bruises from the fall itself. Covering his head with the bag holding his friend's gift, he cowered as loud thuds seemed to get closer to him; each one, in reality, the crack of a super-powered fist slamming into its target.

The fight was between two 'fated-foes,' for lack of a better descriptor, All-Might and All for One. The villain might not have the raw power of the world's symbol of peace and justice, but what he lacked in strength, he made up for in versatility and clever usage of his multitude of quirks. As All-Might rained hundreds of earth shattering punches at the villain, a fan of his might find themselves terrified if they could see him right then. Normally, he had an implacable smile on his face, a smile that showed the world- "Fear not, for I am HERE!" At that moment though, the smile was gone, his visage stuck in an unnatural snarl as the intensity of his stare has a weight similar to the force of his punches. Before him was the man that had caused so much pain and suffering throughout his life, killed his mentor, the previous holder of the title One for All, and even now mocked his heroic ideals.

Grunting as a black orb of energy crashed into his side, All-Might forced himself to grin slightly as he feels he has the upper hand. "Give it up, All for One. Surrender now and I'll make sure you only serve one life sentence." Almost as if to exemplify his point, he lands a staccato of punches into his foe's gut, the smaller man doubling over on the giant's fist.

The masked villain chuckled lightly, his voice only catching slightly from the pain of the blow that could have easily destroyed buildings. The fact he didn't go flying or die from the attack was a testament to his toughness from all his combined quirks. "While your offer is quite generous, All-Might, I feel I must decline. Besides, should I really be your main concern, when there are so many civilians in imminent danger!" Waving an arm across his body, he uprooted a telephone pole behind All-Might, sending it flying towards a couple slouched on a car, the couple of course being the two Midoyira parents.

Cursing to himself, All-Might leaped into action, getting in front of the car in time to slap the projectile out of the way. "Resorting to dirty tricks? Silly me to have expected-" He was going to say more, but the enraged man was cut off as an entire I-beam was sent through his chest. The blunt edge of the iron managed to pierce him, destroying much of his upper chest and missing his heart by scant inches. The couple behind him weren't as lucky. While Inko had only been unconscious, Hisashi had expired from the initial collision with the car, his skull having cracked on impact at such an odd angle. Inko would soon join him as the I-beam exited All-Might, tearing off an arm of hers before impaling itself in the sidewalk.

All for One's laughter was much louder and mocking than before now, as he slowly descended towards the injured hero, feeling he'd won. "See what your ideals get you, young man? Nothing but an early grave. Now, I think it is time for you to die." Holding up a hand, he floats just out of reach of All-Might, charging a larger blast that the one he'd hit him with earlier. "Goodbye, my foe."

All-Might was near to death, there was no denying that. His left arm felt near useless right now, the muscles supporting it not just destroyed but blatantly gone. His eyes though had the same determination as he'd had a moment ago, marred only slightly by the stream of blood leaking from his mouth. "If you think I'd let something like this stop me, you don't know what it means to go beyond one's limits." His eyes locked with his tormentor's eyes, he hooked a foot under the pole he'd slapped down just moments ago, kicking it up to his still-functioning hand. "TO BE, PLUS, ULTRA!" Using every ounce of strength he can summon, he swung the pole, not into the man, but through him, sending the black-clad villain flying, as even now he could hear the other heros and emergency services coming.

"Dang. Gotta stay awake. If I let myself fall asleep, I don't think I'll be waking up." Yagi stood tall, waiting for help as he heard the crying of a child behind him. Despite his drooping eyes and knowing he's in terrible shape, he couldn't help but strain to hear if the child was okay.

Izuku had avoided being hurt by the fight, by some act of divine intervention. The scary noises now gone, he scampered out from between the cars, his eyes panicking and searching for his parents. He didn't need to look long, though, before he heard his mom.

"I-Izu…" Her voice was weak, so weak he almost didn't hear it, but he came running to her. The young boy was almost struck frozen at the sight he found when he rounded the car. His mom and dad were on the ground, both leaning up against the car that All-Might was on the other side of. The way they were is what almost stopped him dead, with Inko's eyes unable to focus and her arm missing, little more than a stump left. Technically, she was better off than her husband, whose chest wasn't moving anymore. He can feel her quirk pulling on his shirt as her other hand was clenched tightly with Hisashi's, the woman clearly not knowing her husband was already long gone. "Izu, come to Momma," she managed to force out, her pull on his shirt fluctuating with every passing second.

Izuku didn't run to her so much as launch himself at her, clinging tightly to her shirt. "Momma, your arm!"

She attempted to lift her hand to comfort him, but she couldn't find the strength to lift the traitorous limb, having to settle for resting her forehead on his. "Shh, everything will be okay, sweetie. You're okay, and that's all that matters." The woman can feel she hasn't long for this world, silent tears streaming down her face. Not at her own death, but at leaving her son all alone. "Baby, can you do something for Momma?" she asks, wanting just one last thing from him.

"What, Momma? Anything, just don't go!" He shouted, understanding that she was likely going to die.

"Can you keep smiling for me, Izu? I just want you to be happy." She managed to get out, offering him a weak smile of her own as her quirk's hold on his shirt weakened further.

Izuku fought back his own sorrow, forcing his mouth to form a large smile for her. The only thing showing it wasn't real was the water freely flowing down his cheeks. "L-like this Momma?" He managed to choke out, his own body betraying him as he wanted to break down and cry for real.

Giving him a small, weak smile, she barely managed to nod her head. "Yes Izu, just...just like that…" Her head slowly tilted forwards before slumping down, her muscles no longer supporting the weight. She'd breathed for a moment, but then it stopped, the pull on his shirt leaving him with her final breath.

Not hearing his mom breathing anymore, he shook her, trying to elicit a response from her. "Momma? Momma?! Momma!" As the reality set in, he broke down, his promised smile crumpling along with him as he cried into her, his whole body shaking.

All-Might, unable to help or even comfort the boy, was little better, tears freely flowing down his face as the pain of being unable to save them easily dwarfed the pain from the iron in his body.