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Things are chaotic for a while. Sasuke is very on edge for the first few weeks. After all, his insane best friend had somehow drawn the ire of one of the deadliest ninjas known to history by besting him in combat and subsequently kidnapping him, and is now hiding from said ninja in the home of an even more deadly ninja whom they are pretending to be related to. Just once, Sasuke thinks, he would like a nice, normal retrieval mission. They could stage a nice, normal coup or fight out a nice, normal civil war somewhere. 

He bets Naruto doesn’t have to deal with this shit.

But the days come and go, and Senju Tobirama doesn’t break down their doors. Madara, true to his word, doesn’t let them out of his sight for even a minute. They move into the rooms next to his, they get supplied with Uchiha robes in various ancient-looking cuts, and every time they sit down as a ‘family’, the man seems to positively glow. Izuna cooks for them, plays Go with Shikako in the evenings, gives Sasuke sharingan advice, and neither of them suspect for a minute that the insane, contrived conclusion they had somehow come to about his and Shikako’s heritage is a ruse. 

It was all settling down very nicely, he thinks.

Shikako works on the seal to return them at night, where nobody can see them — when Madara had seen the seals on Shikako’s skin for the first time he had nearly screamed, thinking Tobirama had done something terrible to her, before they could explain that she was some self-taught genius. Apparently, seals on one’s own body weren’t the done thing. 

There had been some near misses like that, for all of them. Sasuke doesn’t know most of the holy days and festivals and traditions the Uchiha observe, knowledge lost to Itachi’s rampage, but any time he trips up can be blamed on his fictional backstory. On Uchiha Tajima and poor hypothetical Uchiha Nakano’s tragic neglect of their bastard children. 

That trick works like a charm. Every time he ‘confesses’ to some gap in his knowledge, Madara hugs him and does his best to show it to him. It’s… nice. 

This whole thing, being part of a living clan again — even under false pretenses — it’s. Nice. Even Shikako living with him, wearing Uchiha clothes, calling him brother, is it’s own kind of soft fuzzy feeling he tries not to examine. 

He thinks, sometimes- But they both have bonds back home. Shikako has a whole family, and even Sasuke has the rest of Team 7. And beyond that, Team Gai, Ino, his ANBU teammates, even Shikako’s welcoming parents. So many people he misses and- urgh, and likes and hadn’t even noticed had become part of his life until they hadn’t been there anymore. He can’t believe this. This is all Shikako’s fault. And Naruto’s, with his constant talk about how great people and friendship and all that nonsense are.

He will never forgive them. People are awful. 

…Some less than others, he supposes.

In any case, Shikako works diligently on the seal. She says, one day by lamp-light, midnight fast approaching, that it will take a while. She sounds apologetic, too, like interdimensional time travel should be something to be solved within a week. She is ridiculous. He tells her so. 

“It’s not my fault,” she grumps, probably intentionally misunderstanding, “it’s just that there is probably an infinite amount of parallel universes and they probably keep splitting into more.” 

This, to Sasuke, sounds like a cursed form of knowledge. Then Shikako continues, absently, “Don’t tell Tobirama, it would mess with his Hiraishin,” and Sasuke has to both recall that ‘telling Tobirama’ is a thing he could theoretically do now, and that Shikako apparently held the power to render people’s techniques useless by changing their understanding of the universe. 

“He might get lost,” she elaborates and Sasuke amends his previous thought. Shikako had the power to actively turn people’s techniques against them by changing their understanding of the universe. 

He hums thoughtfully, and settles down to doze while Shikako keeps working on the seal. In an hour or so, he will prod her into going to sleep, he thinks.

“It’s all very interesting, multiverse theory,” Shikako rambles on quietly, more to herself than to Sasuke. “Like what qualifies for a split in the timeline, what prompts it. If it creates an entirely new universe or if there are an infinite amount of mirrored universes already and any ‘split’ just means a universe going out of sync with the rest. Is there a timeline accounting for every possible choice anyone has ever made, or just an infinite amount of key events. If we think about theoretical alternate worlds, do they become on some level real? Is there a universe for every story or book that you’ve ever read, and every possible variation of them?” She pauses. “Yeah, probably,” she concludes.

Sasuke thinks about that for a second and hopes that if anything goes wrong upon their return, they won’t end up in one of Sage Jiraiya’s trash book universes. That would be horrific. He is not sure he could forgive anyone involved. 

“Y’know, this might actually be evidence that humans have free will after all.”

Sasuke blinks, suddenly feeling much more awake. “Was that… in question?” He asks.

“Mhm,” hums Shikako, affirmative. “Some people believe that life is just an endless chain of cause and effect, and every decision anyone makes is just the inevitable result of events preceding it, even when we think it’s our choice.” 

Shikako wipes off her brush on a rag, and sets it aside, as she shuffles her notes around. In the end, she pulls two things out of hammerspace, a kunai and a shuriken.

“Let’s say,” she says, lining them up on the table, “there’s an infinite amount of up to this point identical parallel universes in which I’m asking you to choose one of these. It doesn’t matter which one.”

Sasuke indulgently points at the shuriken. Shikako grins. 

“Now do you think that in any of the infinite parallel universes, you made a different choice?”

He frowns. He doesn’t know. “It’s infinite universes, right?” 

“Infinite doesn’t mean every possible thing happens.” 

“Well, but even if I always pick shuriken, I still could have picked kunai instead.”

“Could you? Or would you always pick shuriken, given the same choice and circumstances? And if yes, is it still a free choice?”

Sasuke can feel an existential crisis coming on. To distract from it, he asks Shikako how this is relevant in the first place.

Shikako seals the kunai away again, but doesn’t take the shuriken. Konoha quality. Sasuke tosses it in his hands a few times, experimentally. Are his infinite dimensional counterparts doing the same thing at the moment? Thinking the same thing?

“Right, so we already knew that wildly different parallel universes and pocket dimensions exist, because of things like the summons realm and the pure world, but they probably- started differently. Or diverged much, much earlier. But now we have evidence for universes that resemble ours closely. That have the same starting conditions as ours, or at least we think they do.” 

“Didn’t you say there’s a universe for every decision anyone makes?”

“Well, yeah, but only if people can make different decisions. Like, maybe the timeline splits into one where you chose shuriken and one where you chose kunai, or infinitely many in which you chose either both, or none, or walked away, or did anything else. But if you can only choose shuriken every time, because your decision is a product of preceding events and not actually a choice at all, then there would just be more dimensions that are exactly the same. Right?”

Sasuke nods and tries to picture, again, an infinite amount of Sasukes having the exact same conversation with Kako. If her theory is correct, he hopes there is also an infinite amount of universes in which he is having a wildly different, less strange discussion with Shikako.

He resolutely ignores the implication of universes in which he is not having any discussion with any Shikako at all. 

“But we disrupted this dimension, which seems to have run entirely similar to ours before that. So if all dimensions were the same, some other Shikako and Sasuke should have disrupted our dimension a long time ago, which would have led to Izuna surviving and all of us being a bit different, or not being at all because the timeline changed too much.”

Hadn’t Shikako mentioned something about potentially destroying the universe, when they’d first landed here?

“So,” he tries to follow, “there must be some worlds in which we stumble into that seal, and some in which we don’t?” She nods. 

“Free will,” she grins. “Maybe, at least.” 

“So the timeline could have already split in our original universe, but we didn’t split with them, so we can only return to one of the dimensions?”

Evidently, Shikako hadn’t considered that. She blinks at him for a moment. Huh-s. There’s a smug grin tugging at his lips, in spite of the troubling considerations. Sasuke might not be some mad genius like his team-mate but it’s good that he can surprise her, too, on occasion. They’ll solve it, anyway, somehow. They’ll get back home eventually, wherever in the multiverse that might be. They’re Team 7 after all.

Shikako grimaces, though. “Well, I’ll try to think of something,” she concludes. “But it’s really gonna take a while. We might end up closer to where we left, time-wise, so we probably won’t get yelled at by everyone, but from our perspective it’ll probably be months.”

Sasuke snorts. Months, to crack interdimensional time travel. If Tobirama didn’t try to murder Shikako the next time they saw each other — never, hopefully — they would get along like a house on fire. Or, more likely, a planet on fire. What do you call two mad scientists in a room: A catastrophe. 

“It’s fine,” he reassures Shikako, who still seems troubled. “Besides, it’s kind of interesting, being here, don’t you think?”

Finally, he’s coaxed a smile back onto her face. “Yeah, kind of,” she nods. 

They spend a few moments in comfortable silence, Shikako packing up the last of her notes for today. It’s warm, because Uchiha houses are always warm, and because Shikako’s seals can be very nifty sometimes. 

Madara and Izuna should be thankful for the silencing seals on their walls, he muses. He bets they never have to consider the vastness of the universe. Multiverse. Whatever.

“If all else fails, we’ll go to the Dead Wastes and wing it from there,” Kako says. 

“Why,” he can’t help asking, “What’s there?”

“S-Rank secret.”

There is a garden there now, in the Wastes. Sasuke had heard that Shikako died twice, once on the operating table and once before that. Naruto had spoken of a temple. They had all seen the beam of light, beyond the horizon. Shikako, who knows things about the universe no sage he knew had thought of. Had she learned them there, those things about life and death, time and space, will and choice? He bites his lip.

“Let’s avoid that for now,” he finally says. 

He trusts his team mate, his best friend. But he doesn’t trust her not to do something risky if she thought he wanted to go home more than he wanted her to go home with him.

They go to sleep. The next day, they don’t talk about the universe. Instead, after breakfast, Shikako lets herself get coaxed into sparring with Madara. Losing is a given and she takes it with grace. Sasuke watches, and when he sees Madara coming at his teammate, Mangekyo Sharingan blazing, he barely even has to stop himself from using the replacement technique on her. Progress. 

Madara has taken to lamenting — in few words but with triple the emotion — over Shikako’s abject lack of training in the Uchiha style. When she has to cautiously explain that her chakra element is earth and she wouldn’t benefit much from knowing a fire-user style, he immediately jumps into reassurance that no matter her chakra nature, she is still an Uchiha and what’s more, she is still and will always be their sister.

It’s very touching. The dark terror of the shinobi world, first missing nin Uchiha Madara, can be surprisingly sweet. In a stupid way. 

In the beginning, Shikako had been very reserved about Madara. Sasuke had thought that it was because she worried about her non-Uchiha heritage, but she’d been fine about Izuna and everyone else they had met in the compound. Just Madara seemed to, almost, scare her. 

They hadn’t talked about it, Shikako unwilling to explain and Sasuke unsure of how to bring it up. It had just been- surprising. To find that Shikako was treating someone the way their reputation dictated instead of welcoming them with open arms and an even more open mind.

Normally, she couldn’t stop meddling. Hell, she was friends with Gaara

It had worried Sasuke, to be honest.

But now, watching her let Madara guide her into the beginning stance of the Uchiha martial style, Sasuke thinks perhaps some problems truly do resolve themselves if he leaves them alone long enough. Madara, at the very least, looks at Shikako like she hung the moon and stars. He hadn’t actually stopped doing that from the first time he’d seen her, a tired intruder mess in Sasuke’s room.

They had been so lucky he hadn’t killed them. Sasuke cringes even thinking about it. 

This would be good, though, if they ever ran into Senju Tobirama again. Sasuke knows better than to take Shikako at her word when she had said “We took a little trip — I slapped chakra suppression seals on him to keep him from murdering me but we actually got along great! — until I figured out how to drop him off at the Senju compound again. He’s probably forgotten all about it by now.” Even if it was the Nidaime, if he tried to harm his team mate in any way, he would be tasting Amaterasu. 

Not that Senju Tobirama would try anything in the Uchiha compound, and Sasuke and Shikako had very few opportunities to leave without a clingy Madara or Izuna insisting he accompany them. This, too, was probably a good thing for now. 

Hesitantly, Sasuke is content with the way things are going. Not that he would admit it.

Of course, a few weeks later, Shikako gives into her instinct to meddle and, of course, kicks loose an avalanche of chaos. 

Upon finding out that bloodline theft was a very serious problem in these times instead of a fringe concern reserved for weirdos like Orochimaru and illegal Hidden Cloud government divisions, Shikako presents Sasuke with the anti-eye-theft seals he had asked for. They are very functional, preventing any unauthorized chakra use or surgical interference on his eyes, even when he sleeps, and destroying them if anyone forces through the first few protections. It is entirely possible that someone trying to use chakra on his eyes will catch on fire. The seals even protect against physical damage to some extent, although Shikako warns him not to test that. 

Oddly enough, she empathises that he can disable the seals if he wants to, both to get them healed and to have them taken out, if he ever wants to. She also rambles about thermal vision, lasers, and some kind of ocean shrimp that can see, like, twenty colours humans cannot perceive, but he is too distracted by the design on his face to pay much attention. 

Privately, he had kind of feared that the seals would make him look like Gaara with his thick racoon eyeliner, but they are barely visible. Tiny, intricate characters, almost blending in with his lower lashes. The design had covered his whole face and most of the floor originally, but Shikako had simply shrunken it down.

In their fervor, though, they had forgotten an important thing. To Uchiha with the sharingan on, seals are actually kind of obvious. So naturally, both Madara and Izuna freak out. And once Sasuke gets them settled down enough to explain, they freak out again, in a different way. So, long story short, Shikako spends the next week painting seals on about three-hundred Uchiha’s eyes. 

That’s when, completely in line with his previous behaviour, Madara decides that Shikako must be protected at all costs. 

He tries to get Sasuke into it, too. “Your sister is brilliant,” he says one day over tea while Izuna and Shikako were out watching a fire dancing rehearsal, and Sasuke agrees. Truthfully, Madara’s obvious appreciation and admiration of his ‘little sister’ had won him serious brownie points with Sasuke. It’s a rule of thumb: People who like Shikako could be trusted, people who didn’t could be set on fire. It hasn’t failed him yet. Madara, trustworthy, continues: “But she is not a strong fighter.

That, Sasuke does not agree with. At all.

“You have to stick close to your sister, okay?” Madara continues, oblivious to the question marks forming over Sasuke’s head. “We’ll keep her as secret as possible, but there are always leaks. Other clans could — will — come for her.”

“Hm,” Sasuke says, in thought. “That would not end well.”

“Yes,” Madara agrees, “none of us want to see her harmed again. And her knowledge in the wrong hands could be catastrophic.”

“No, I meant for them. And the landscape.”

Madara manages an owlish blink at him. Sasuke snorts. How should he put this…

“Do you know why I win most of our spars?”

The clan leader shakes his head, confused. It looks very funny with his untamable mop of hair. Maybe Sasuke is having too much fun with this.

“It’s because most of Kako’s techniques are too deadly for a friendly fight.”

Explosions don’t usually lend themselves well to fighting people you want to remain un-exploded. Of course, other reasons Kako can’t fight to her full potential are because Nara shadow techniques would fully give the game away, and revealing the full extent of her sealing knowledge would be tactically unwise. But he can’t explain that. Madara gapes. 

What’, say his eyebrows, up to his hairline. Sasuke almost smirks in satisfaction.

“Against normal ninja, Kako can hold her own as well as I can. She wouldn’t appreciate being condescended to like this.” 

Madara looks suitably chagrined. He nods, still a bit dazed by this new information. Sasuke understands: Shikako doesn’t really look as dangerous as she is, and Madara first met her as Sasuke’s tragically dead little sister. Those associations are hard to shake.

Then, as Madara makes to leave, Sasuke remembers something. Senju Tobirama, to be exact, who might be planning bloody, scientifically unethical vengeance on Shikako and what she had charitably termed ‘their little trip’, at this very second. Sasuke should take precautions, against such a threat. Especially when the opportunity had just presented itself on a silver platter.

“Wait,” he calls to Madara. “Kako is strong, but… against people like- like that Senju. I worry.” He even adds his best frightened expression, which is little more than a slight widening of his eyes but it seems to get the point across.

Madara’s back straightens, relief and determination. 

“Don’t worry, brother,” he says. , not otouto, because he knows Sasuke hates the word. “I won’t let anything happen. To either of you.”

Sasuke doesn’t have to fake his grateful smile even a little bit. Shikako will kill him for this.

Madara… won’t go out to murder Tobirama unprompted. Probably. Sasuke hopes, at least. Or Shikako might actually maim him.

The next day, a peace treaty comes, bearing the Senju crest. Sasuke has to hide a grin behind his sleeve.

This will be… absolutely hilarious.