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Senku derived two experimental results from the (accidental) explosions.
First, that they'd made actual, live gunpowder from scratch, which was pretty exhilarating. It could give them a fighting chance against Tsukasa, if they had a little more time to refine it.
Second, that they had no more time. There was no way Tsukasa hadn't heard the sound, seen the smoke, and depending how close on their heels he'd been pursuing, he might be upon them at any moment.
Sure enough, no sooner had he sent Taiju and Yuzuriha away on a list of made-up errands, did a bulky shape crash out from the forest—and charge straight at Senku. That monstrous strength plucked him up by the collar, so that his feet sickeningly left the ground, finding nothing but empty air no matter how much they flailed, so that his desperate hands could find no purchase but the tree trunk arm that held him easily aloft.
Dangling helplessly from that unforgiving grip, staring into the face of his imminent death, a death named Tsukasa, Senku was left with few regrets. Only one, really.
In science there were a lot of post-mortems (or in this case, slightly pre-mortems), analyzing which wrong step had led to the experimental failure. Was it reviving Tsukasa in the first place? Restoring life to the one that now sought to take his? But he couldn't regret it; calling on Tsukasa's strength had clearly been the right move at the time, no matter where it left him now.
Trying for the gun and gunpowder had been the right move also, so he didn't regret that, nor the direction they'd set out in to achieve it. It had been a gamble, a race, one that he'd lost. But no matter how he looked at it, he'd made the best possible choices all the way here.
The only remaining regret he felt was this: that he might die without having shown Tsukasa the truth that was so obvious to him. That he had never managed to bring Tsukasa around to the wonder of science, how it might have made equals of them, the consummate fighter and the scrawny scientist, if only he'd had a little more time.
Tsukasa dragged the recipe for the revival fluid from him, bartered for a sincere promise not to go after Taiju and Yuzuriha next. Tsukasa was always so sincere about things. Then there was nothing left but the inevitable.
"Do not feel regret, Senku." Tsukasa's too-perceptive eyes skewered him from an arm's length away, somehow hearing his thoughts even unspoken. "Death comes for us all. I did not wish to end your life, but in this brutal world, even the brightest flame could be extinguished at any moment. Death is the great equalizer, the ultimate fairness. We will all meet there in the end; I'm only sending you on a bit sooner."
"That's not right." Senku scrabbled at the arm holding him. Taut muscle and pronounced tendon bulged prominent as steel cables. He might as well have tried to knock over a suspension bridge while dangling from its deck. "It's not death, it's science. Science is what makes us equal. If you'd only let me show you."
"No, Senku. Science is a tool, to be held over our heads. A weapon. Speaking of which, do you have any more revival fluid left?" Tsukasa's voice was soft, pleasant. "If you do, I'll take it from you now. I don't want it to shatter when I kill you. That would be an even greater waste."
Senku's hand went to one of the pouches at his waist. He wasn't trying to hide it from Tsukasa, who already had the recipe now, and would have this from him one way or another. Taiju and Yuzuriha still had more, so that wasn't a problem. The question was whether he should hand it over whole, or bargain something else from him for it. Letting it smash into the ground would be a waste indeed. Collecting nitric acid was a pain in the ass.
Tsukasa was more impatient than Senku had thought, because he began to shake him like a rag doll, as if hoping the loot would just drop out of him with some force.
"Give me a minute to think, would you?" Senku snapped, but he took out the last jar of revival fluid anyway. To his surprise, Tsukasa wasn't even looking at him, instead gazing wild-eyed at their surroundings.
He wasn't shaking Senku at all.
The entire ground was shaking, the rocky surface of the cliff juddering like the shiver of the world.
Even the tops of the pine trees in the distance were swaying as violently as grass in a gale. And farther out, higher up, there was a distant cloud of dust. Almost like—
"Rockslide!" Senku swore and jerked around, but nothing else had changed about his situation, and he still got nowhere. "The gunpowder explosions— This region is prone to avalanches, or at least it was, back in our day— Tsukasa, you gotta—"
What Tsukasa did was reel him in and upwards. Even though he'd been seconds away from killing him, Senku was pretty sure the movement was meant to tuck him closer, to carry him to safety, rather than winding up to yeet him into the rockslide, and let it do the job for him. But Senku was still flailing and the ground was still shaking and their independent trajectories ended in collision: a ringing pain as their heads knocked together, compounded by the caustic sting of the revival fluid smashing into both of their faces.
At once, there came a sudden, queasy shift, the world rotating around him as though he were the center of a turning wheel, the axis of a wildly spinning planet. And then he was looking down at himself. Holding a person by the shirt, fabric bunched in his fist, but that person could have been his own double, face like a mirror, hands clutching at his wrist and some shards of broken pottery, familiar eyes burning with an intent rage that he'd never seen in his own reflection.
That couldn't be right. How could he be looking at himself—
"What did you do?" Tsukasa, in Senku's body, snarled.
"You think I did this? I'm a scientist, not a wizard! More importantly, there's a freaking rockslide. What are you going to do?" Senku was caught off guard by how strange his voice sounded, how it rumbled in his chest and throat, and it took him a beat too long to recall he was the one in Tsukasa's body. Forget the how. If anyone was going to get them out of this, it was going to have to be him.
"You are so… ridiculously tall, Tsukasa," he grumbled, taking in the surroundings even as he took off at a run. The change in perspective was dizzying, and he felt like he had to be on stilts somehow, or floating above the ground. Not to mention how incredibly powerful his muscles felt, his lungs, all the parts of his body working in perfect concert like pistons in an engine. Each stride felt simple, natural, effortless, even with Tsukasa clutched in his arms and an avalanche bearing down on them like a deafening thundercloud. Maybe he really was floating, running on air. The distance in front of him shrank with comical speed, even as bits of gravel and detritus were already beginning to skate in front of his path, kick up at his ankles and shins.
Every time he looked up, he was put off by how different the world looked from when he'd last seen it—and that was discounting the oncoming avalanche. A slightly taller viewpoint revealed details that simply hadn't been there before: a yellowish crust atop the boulder he'd sat against at lunch meant sulfur for sure, they could have harvested that for more gunpowder. And through new gaps in the treecover, he could see a tempting offering of plants ripe for harvest, easily within reach: the glossy broad leaves of kuwazuimo, dense kudzu vines and leaves, and even a huge-ass patch of shiitake that would have done worlds of good for said lunch, a stew thin and bland and sad.
"Hurry, Senku! The ground is breaking," said Tsukasa urgently, and Senku spared a glance behind them. Right, now wasn't the time for sightseeing and foraging, even if it was unfairly easy from Tsukasa's vantage point. Dark, angry cracks already split the rock where they had just been standing, jutting forward another few creaking zigzags at a time as he watched. He wouldn't be surprised if that whole cliff crumbled away: if not now, then when the landslide reached it. He didn't plan to be here when that happened. He just also couldn't run toward the landslide either, which would only knock him flat and drag them back off the cliff.
"Need to move laterally." Senku tried to dig up everything he'd ever read about rockslides from memory, even as he contended with moving in completely unfamiliar ways in a body strangely familiar with it. "The layered volcanic material makes the ground here naturally unstable… Three gunpowder explosions was too much… Debris flow seems to be approaching at approximately… judging distance by those couple trees… 20… maybe 26, 27 km/h. Fast, but could get faster yet. If there's water, we could have a mudslide on our hands… not to mention the air blast… We're not going to make it…"
"Get behind something," Tsukasa urged. "If you can't get out of the flow, at least duck for cover."
Not a bad idea. Just as the front of the roaring wave reached them, Senku threw himself behind the widest, sturdiest tree he could find, and braced Tsukasa between himself and their makeshift cover. It was only the thinnest early part of the landslide, and thankfully it split around the tree, flowed past them and rejoined, with only a few loose rocks striking him in the arms, legs, back. But it was growing. Even by the sound of it alone, he could tell it was growing. The tree wouldn't hold.
"We can't stay here. But there's nowhere to go." The bulk of the rockslide was rumbling by to their left, sending up choking clouds of dust, while their right was cut off by the abrupt cliff's edge, followed by a perilous ravine. "Let me think— Can we swing across— Build a bridge—"
"Senku, just jump over it!" Tsukasa was knocking on his chest, so light Senku barely felt it. It occurred to him that Tsukasa was used to watching his strength around others. Now in Senku's body, he was barely applying pressure.
Senku stopped his frantic calculations anyway to stare at him. "Jump over what? The freaking Grand Canyon over there? Are you crazy?"
"I can do it, easily. My body can, anyway." The distance looked impossibly wide to Senku, but Tsukasa looked equally incredulous that Senku thought so. Reflexively, Senku held Tsukasa just a bit more tightly, in case he had a mind to break away and try it himself, in a body that was guaranteed to fail.
"I guess you should know best," Senku said skeptically, still waiting for Tsukasa to say he was making a seriously unfunny joke. But the landslide crashing around them was getting louder and heavier, and more and more rubble was striking him. Sooner or later, something bigger would come for his head, or the tree would snap, or gravel would knock him off his feet. They had very little time, and even less choice. "Fine, then. Here goes nothing—"
Before he could psych himself out of it, Senku tore out from the tree cover, picking a few protruding boulders to leapfrog between until he made it to the edge. Each jump made him more confident that Tsukasa wasn't bullshitting him after all, soaring over the churning rockslide like it was child's play rather than the single most terrifying experience in his life—and that was considering he'd been facing his murder just a minute ago.
The power in his legs was incredible, taking him over great leaps that would have left the real him winded. Upslope, the roar of the rockslide was picking up. Below, the chasm only seemed to yawn and grow the closer he got to the edge of it, the ground a dizzying drop so far down he couldn't make out more than a vague orange streak, to suggest there might be terrain down there, rather than a bottomless drop.
When you swapped to a new character in a video game, it made sense you inherited new abilities, right? So he gripped Tsukasa in his arms, and jumped.
He had expected to duck and roll on the other side, another unfamiliar action he was sure to take clumsily, but he landed easily on his feet. Not a moment too soon. Back on the other side of the chasm, the landslide picked up in volume, piling over the side of the cliff with a force sure to have swept them off with it, and down into that unknown drop below.
In his grip, Tsukasa was starting to struggle again, but it was shockingly easy to keep him aloft. With his feet off the ground, his armspan shorter than Senku's, Tsukasa could do nothing, reach nothing, but Senku.
Senku knew how that felt. He lifted his other hand to help steady him, and Tsukasa flinched back. He knew better than anyone the strength in those fists.
Right. They had been in the process of killing/being killed. The giddy relief evaporated from Senku's system, to be replaced by a sudden surge of tension. There was something in this new body that seemed particularly attuned to conflict. The prospect of a fight made the blood sing in his ears, his heart thump in his chest, hyper aware of the potential enemy in such close proximity.
It occurred to him that his light grip might actually be hurting Tsukasa. Embarrassing, when even in the process of killing him, Tsukasa had been much more careful with his strength. Hastily, he set his passenger down, and was unsurprised when Tsukasa instantly backed away from him.
"Now that the crisis is past, tell me." Tsukasa looked at him warily, then over his shoulder, searching out an escape. "How did you manage this?" He gestured between the two of them, and it was strange how he used Senku's limbs, with a crisp precision, attuned to the physicality of each motion.
"I told you, I had nothing to do with it." Senku could barely believe it himself, even now that he finally had time to think about it. In a world where humans could be turned to stone and revived, it still boggled belief that they could switch bodies. Senku had the strange sensation again of looking at himself in a mirror, but from a strange angle that made all his features foreign. The upward spikes of his hair were more prominent, his forehead larger than he was used to in reflection, his eyes gazing up at him, hard and hostile, from under his brow. Was that how he looked to tall people?
"The timing is too convenient," his own face said. "Just as I was about to kill you—"
"—suddenly our positions are reversed," Senku agreed, and Tsukasa went silent.
"Guess I can figure what you're thinking, right about now." Senku took a step forward. Tsukasa took two steps back. "Now that you're no physical threat to me, now that you failed to kill me, you're wondering if I'll take advantage of your OP body and fight back."
"With my body and your knowledge, you'd be unstoppable in this world." Tsukasa gave him a scrutinizing look, and seemed to read something in his own expression, which he had to know well. After a beat, he lowered his head in acceptance, baring the back of his neck.
The last fragment of stone that had remained from the petrification, that had been Senku's one plan to rise from the dead, was gone.
Senku stared at it longer than he should have, while Tsukasa held his hands open at his sides, head lowered and resigned, like the inevitable result at the answer side of the equation.
"Now that I'm stronger than you, Tsukasa, you've given up. In the world you want to create, the ones born with physical strength will rule. It's no different than a world with guns, just a different form of power. Is that the world you want? Simply because you'd be the one on top?"
Tsukasa didn't answer. Instead, he touched the nape of his neck, right where Senku had been trying to draw attention a lifetime ago, where the petrification should have remained. "I can give you advice on how to make it as swift and painless as possible. If you wish it."
The sudden image of striking him there made Senku's throat go dry, a strange bitterness flood his mouth. The image wasn't only visual, but visceral, and he could almost feel the way his arm would move, the solid resistance of sudden impact, the even more solid give—
"If you're going to tell me what your next step would've been," Senku's voice came out too loud, as if trying to shout over his imagination, and pretend at steadiness besides, "it's only fair that I'Il tell you mine. Hell, we can play it out if you want."
Already unmoving, a strange stillness came over Tsukasa, making him seem even more inanimate. Nothing Tsukasa did looked right in Senku's body: spare and deliberate in gesture as he'd been in his own, every minor movement controlled and accounted for. After a moment of thought, he straightened his neck, and opened his eyes. "I'm listening."
"In the first pouch at your hip, closest to your right hand, is a white powder. Yep, you can go ahead and touch it. On our way over, we came across a bunch of kuwazuimo taro. Taiju, that idiot, wanted to eat it. Didn't anyone ever teach him the difference between edible taro and the toxic kind? You know better, right? That stuff is full of calcium oxalate crystals, basically microscopic needles. Useless for food, but if you dry it and grind it up, it's basically nature's pepper spray. If I smashed it into your face, you'd be writhing in pain for hours, long enough for me to get away scot free."
Ever ruthless, Tsukasa's hand clenched around the bag, and Senku gave a tight smile.
"Now, keep in mind, I don't want to be pepper sprayed. So now that I know you know, and I know what hell that would wreak on my eyes and throat, I don't actually want to get into a fight with you. And you don't want to get into a fight with me. Because I could overpower you. And you could put shit in my eyes that would make me want to scratch them out."
Tsukasa picked up quickly. "You're speaking of de-escalation, as a way out of our conflict. But remember how well that worked, back in our world."
"It somewhat worked. We haven't had any major world wars since our weaponry advanced to the point of mutually assured destruction. It's the same way here. If one person is stronger than everyone else, he can overpower them and do whatever he wants. If everyone is armed with science, then there's less incentive to fight. If it doesn't work, we just need more science, not less. That's all I'm saying. I don't want to arm one person and give him power over the masses. I want us all to be on the same playing field."
He was sure Tsukasa would get it, would see it his way, if he only thought about it more. They'd already traded physical viewpoints, literally. Now he just needed the stubborn man to take one step farther.
Instead, after thinking it over, Tsukasa said slowly, "If you had this in your back pocket, why didn't you use it? I was about to break your neck. A moment later, and you would've been dead."
When Senku didn't answer immediately, Tsukasa studied the fingers he'd dipped into the pouch—then stuck them in his mouth without hesitation.
"I literally just told you about the toxic—"
"Sugar," Tsukasa cut him off, with a nasty look. "This is just sugar."
"Okay, yeah, I was bluffing." Senku held his hands up. "Nothing I said was a lie, though. We did see kuwazuimo on the way over, Taiju really did almost try to eat it. And if I'd had the time to grind it up into pepper spray, it really would have had the effect I said. But I was more focused on the gunpowder, and then you found me sooner than I thought."
"So there's nothing stopping you from actually killing me now," said Tsukasa flatly.
"That's still my body. If we don't know why we switched, who's to say we won't switch back someday? If that happens, I want my body to still be healthy and, you know, alive."
Tsukasa thought this over too, and then gave a final, whole-body sigh. "And I suppose, by the time we do, you'll have perfected your gunpowder. You'll have armed this pure world with guns."
"Is that so bad, Tsukasa?" Senku cautioned taking another step closer, and this time Tsukasa didn't retreat. "If the strongest person didn't automatically rule it? If everyone had a fighting chance?"
Tsukasa touched the sugar pouch again. He was doing things with Senku's face that made it entirely strange, pulling it into the tight scowl of one facing an unwelcome truth. "I don't know that I fully agree with your views, but it's clear you've beaten me, one way or another. I will work with you to restore us to our proper bodies. And to build this world according to your vision. By strength or by science, or by simple unexplained luck, you have won that right. I cannot argue with that."
And as he crossed his arms, shoulders imperious, chest straight, looking for all the world like a miniature version of his old self, ready to fight, Senku finally sat down hard beside him.
"Good!" He exhaled a breath he'd been holding, and sprawled back onto the ground, to Tsukasa's alarmed squawks. "Being you is overpowered as hell, but it's exhausting!"
He let his eyes drift closed, fully confident Tsukasa wouldn't do anything underhanded while he was out. Maybe he'd wake up back in his body again, or maybe it would take some more diligent science and experimentation to figure that out. But if he could win Tsukasa to his side, nothing seemed quite so impossible after that.