Prologue

Dungeons appeared in this world. However, the dungeons themselves only ever impacted half of humanity.

This was because the only people capable of entering them were women.

Actually, that isn’t quite the accurate way to put it. Men could enter dungeons. They were physically capable of stepping inside, but the very concept of "Levels" simply didn’t exist for them.

Defeating the encroaching monsters, gaining experience points, and leveling up to become stronger—that game-like progression only applied to women.

Without levels, you can't get stronger. In short, men were doomed to remain weak.

Until a few years ago, sports like baseball and soccer were dominated by men, but once the dungeons appeared, everything was flipped on its head.

Men stood no chance against women who had braved the dungeons and raised their levels.

However, as if to serve as a substitute for levels, a different concept manifested specifically for men. That concept was "EXP."

Once a boy reached puberty, a numerical value would appear somewhere on his body. It could be a thousand, ten thousand, or maybe just one. That was his "EXP."

Of course, this isn't some gruesome tale where women kill men to harvest their experience points. It isn't that violent.

"Just like how water moves toward the side with a higher concentration in saline or sugar solutions... This principle applies to other things as well..."

In fact, far from being violent, it was actually quite... erotic.

The method for a woman to obtain EXP from a man involved things like holding hands, kissing, or hugging. It required the kind of intimate physical contact usually reserved for lovers.

"There are different types of semipermeable membranes. Take slugs, for example—you all know them. Their bodies are encased in a semipermeable membrane, so if you pour salt on them, the moisture gets sucked right out. The same thing happens with sugar. And actually, something very similar happens to humans."

In a dungeon, you have to fight for your life to gain experience. However, you can also gain it by getting "busy" with a man.

There’s no risk of death, and it feels good. It’s a win-win situation.

"...And so, by plotting the graph of the exponential function, we've determined that the amount of transferred EXP increases accordingly. Now, with that understanding in mind, try to calculate your own EXP."

Naturally, the more EXP a man has, the better. Men with over ten thousand EXP are incredibly popular, and those with over a hundred million are treated like super-idols. Actually, at that point, they’re practically at risk of being assaulted.

There is no way to resist a woman who has leveled up in a dungeon and even mastered magic or skills. Apparently, men with EXP in the trillions are immediately taken into government protection. They get to live a life that is, by all accounts, quite enviable.

"Each of you, plug in your own EXP and determine how much transfer would occur during certain acts... Hey, Mura Tatsuya, are you even listening?"

"Ow!"

Before I knew it, my teacher was standing right next to me, smacking me over the head with a textbook. She looked down at me with sharp, narrow eyes, her ponytail swaying.

The hand she’d used to smack me was her right. She didn't have the other one. Apparently, it had been eaten by a monster back when she was a dungeon diver. She’d retired after that and become a teacher. That said, she was still a young woman of twenty-five.

”Sensei, that’s corporal punishment," I grumbled.

"Oh, really? Why don't you try suing me, then?" Sagara Chie-sensei replied.

She didn't seem bothered by my words at all, giving my head one more tap with the book before heading back to the podium. This was currently Chemistry class... No, wait. It was Health. Which was why the classroom was filled with nothing but boys. Aside from the teacher, of course.

I really didn't get why a female teacher had to handle the boys' Health class. Expecting us to listen seriously was asking for the impossible. Especially when the teacher was a beauty with a killer figure. Her large chest was practically trapped inside her tight shirt, looking like it might burst out at any second.

"In short, when you engage in intense sexual acts involving mucosal contact, a larger amount of EXP is transferred to the woman. At this time, the woman experiences pleasure proportional to the amount of EXP flowing into her. Now that you understand, Mura, don't be shy and do the math. This is important stuff."

She said it was important, but...

"Sagaracche-sensei! Mura doesn't have any EXP!"

Sakakibara, the guy sitting behind me, shouted out a teasing remark. Sagara Chie-sensei. Hence, "Sagaracche."

"...Is that true?"

Men with absolutely zero EXP were almost unheard of. Sensei shot me a doubtful look. I rolled up my shirt sleeve and slid down the arm cover underneath it. On my right arm, the character for 'None'—<None>—was visible. Seeing the mark, which was clear even from a distance, Sensei's eyes widened in surprise.

"Oh... My apologies. I suppose there are people like that. Well, look, EXP isn't everything for a man, so don't let it get to you too much. Just make sure you listen to the lecture properly."

"Got it."

The word <None> had surfaced on my arm when I was a kid. My friends had hundreds, thousands, and occasionally even ten thousand. But I was <None>.

Once that got out, my male friends teased me mercilessly, and the girls started giving me the cold shoulder.

By the way, while the boys were learning about the mechanics of EXP, the girls were taking a class on dungeon exploration. Though "exploration" apparently just meant getting a brief explanation on the first floor.

I wondered if I’d been born a girl, would I be having a grand adventure with swords and magic like some light novel protagonist? As I was lost in these meaningless thoughts, the bell rang to signal the end of class.

"You boys need to keep in mind that you are the 'prey' here. A man cannot defeat a woman who has leveled up. No matter how much you trust someone, never let your guard down. That’s all for today. Mura, come to the nurse's office after school. Dismissed."

As I watched Sensei walk briskly out of the room, the classroom erupted into vulgar chatter. That was just how it went after a Health class.

"Intense sexual acts with mucosal contact, huh?!"

"Damn! I'm already rock hard!"

"He said we're the prey! Man, I want to be eaten already!"

"You only have three hundred EXP, dude. Nobody's gonna eat you."

Dirty talk was the universal language of boys. Even though the second year had just started and we'd only recently been placed in this class, they were already bonding over their gutter-talk. Listening to their voices, I turned my gaze toward the window. Outside, the cherry blossoms were in full bloom, beautiful despite having to listen to all the smut.

"Pardon the intrusion."

"You mean, 'Excuse me.' Whatever, just sit."

When I entered the nurse's office after school, Sensei spoke without even looking at me. She seemed to be researching something on her computer.

Keeping her eyes on the screen and moving the mouse with her right hand, she used her foot to nudge a nearby chair toward me. It wasn't very ladylike, but I guess that was just how former dungeon divers were.

"Is this a lecture about me not listening in class?"

"I'd love to give you a piece of my mind about that, but this is a separate matter."

Once I sat down, Sensei turned her body toward me and looked me straight in the eye.

"Mura. Even though I didn't know you lacked EXP, I said some very insensitive things. First, I want to apologize for that. I'm sorry."

Sensei bowed her head deeply. There weren't many teachers who could face a student so honestly.

"You don't need to apologize. Everyone knows, and it doesn't really bother me."

"Even so. I'm sorry."

"...Right. Well, thank you for being so considerate."

When I answered, Sensei looked up with a small smile.

"Now, it might be a bit awkward to ask this right after apologizing, but... could you let me see it one more time?"

"I don't mind."

I rolled up my shirt and arm cover just like I had during class. The character for <None> was still there. After staring at it for a while, Sensei turned back to her computer.

"It really is <None>. But that’s strange. I know a bit about EXP from my days as a diver, but I’ve never seen or heard of the character for 'None' being used."

"Well, that’s probably because men with no EXP are so rare."

"No, that's not it. Look at this."

Sensei showed me the computer screen.

"Men with no EXP are labeled as <Zero> or <0>. There are no confirmed cases of the character <None> being used. Except for you."

"Huh. Well, I’m definitely <None>."

"...Hey, Mura. Do you really have no EXP? Have you ever... tested it?"

"Teacher. Isn't that sexual harassment?"

"Ah! S-Sorry! Again, I’ve asked something insensitive..."

Asking if I’d "tested it" was basically the same as asking if I’d ever been with a woman.

"It’s fine. Besides, whether it’s <None> or <Zero>, the fact remains that I have no EXP. Is that all? Because I’m gonna head out now."

As I stood up, Sensei reached out toward me in a panic.

"Mura, wait! Let me get a closer look at it!"

"Ah, wait—!"

Before I could dodge, Sensei grabbed my arm.

"Crap! Let go!"

"Huh? ...Ah, ah, ah... Aaaaahhh♡♡♡♡♡♡♡♡♡♡♡♡♡!"

Hearts appeared in Sensei's eyes as she fell backward.

"Sensei! Let go of me!"

I tried to pull my arm away in a hurry, but there was no way I could match the strength of a former dungeon diver. We ended up tangling together and collapsing onto the infirmary bed.

Sensei fell right on top of me. Perhaps because she’d been forced into an awkward position, the buttons on her shirt suddenly popped off.

"Wh—?!"

Before I knew it, my face was buried right between those bountiful mounds. At the same time, Sensei's body gave a violent jolt.

"Ngh, haaaaaaaah♡♡♡♡ Mu-raaaaaah♡♡♡♡♡♡ Nnnkyuuuuuuuuuu♡♡♡♡♡♡♡♡♡♡♡♡♡♡♡♡♡♡♡♡♡♡!"

"Se-Seeenseiii!!"

"O-O-O-O-Oh♡♡♡♡♡♡♡♡♡♡ S-Stop it! Aaaaah♡♡♡♡ T-This is... my first time... feeling like this! Ooooohhh♡♡♡♡♡♡♡♡♡!"

"Senseiiiii!!"

Sensei's screaming voice was actually a lot cuter than I expected.

"Sigh, I really screwed up... I was trying so hard to be careful..."

After that, I’d left Sensei—who had passed out on the bed with her eyes rolled back and a face that definitely wasn't fit for public viewing—and fled back to the apartment where I lived alone.

I heated up some frozen tantanmen, ate it quickly, and headed to the bathroom. After washing my body, I soaked in the tub and rubbed my right arm. The character <None> was there. And further up my arm, more characters followed.

<Muryoutaisuu> (Immeasurable)

That was my true amount of EXP.

When I was little, only the character for <None> had surfaced, so no one besides me knew about this.

I only found out that "Muryoutaisuu" was a unit for a number when I was in my first year of middle school. The characters that had been faint gradually became clear, and curious, I looked it up.

Muryoutaisuu. That is 10^{68}.

To put it in easy-to-understand numbers, it looks like this:

100,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000.

(T/N: Muryoutaisuu is the largest named number in the traditional Buddhist numeral system it is typically defined as 10^68. It also used to represent something that is immeasurable or ultimate)

That is one Muryoutaisuu.

When I first realized this fact, I was shaking like a leaf. Even men with EXP in the trillions are considered national treasures. My amount of EXP was far beyond that. I was practically a global treasure. I expected to be treated like a VIP higher than the US President and live an incredibly luxurious life.

However, that expectation was quickly crushed.

Back when I was in the seventh grade, there was a girl I liked. She had pale skin, beautiful long hair the color of deep navy, and she was a quiet girl who loved reading. Her name was Hamano Yura. We’d been on a first-name basis in elementary school, but after starting middle school and being placed in different classes, we didn’t talk much.

I had a shallow, foolish delusion.

An autumn afternoon when you start craving human warmth. A deserted library bathed in the setting sun. Just her, the library assistant, quietly turning pages in the crimson-stained room.

I’d suddenly enter the library. I’d pick out some difficult, cool-looking literary novel and hand it to her at the counter.

She’d scan the barcode. Tatsuya-kun reads such difficult books, she’d think, her heart fluttering. She’d open her mouth to speak, but being shy, no words would come out.

The book is handed back in silence. As I take it, I’d brush against her fingers just a little. And what would happen then? Usually, just touching hands only transfers a tiny bit of EXP, but my total amount was gargantuan. Even if only 0.001% was transferred, it would be an astronomical amount of EXP flowing into her. Receiving that much EXP, she’d be totally smitten. I love you, Tatsuya-kun! Please go out with me! That’s how it was supposed to go.

After fantasizing about it night after night and running countless convenient simulations in my head, I finally put it into action one autumn day.

I entered the library, found a complicated-looking book, and went to the counter...

"...Ah. T-Tatsuya-kun. It’s... been a while. You read these kinds of books?"

"Oh, yeah."

"Um... It’s due in two weeks. Uh, i-if you want, tell me what you... thought of it!"

"Oh, sure."

She handed the book back with a shy smile. I went for it, thinking now or never, and brushed against her fingertips. That reckless action was my big mistake.

To a girl who had only just reached puberty, an avalanche-like amount of EXP and pleasure came rushing in. "Smitten" didn't even begin to cover it.

"...Ah... Aaaah... Ah..."

"Yu-Yura-chan?"

Her eyes, usually hidden behind her long bangs, flew wide open, and Yura-chan’s entire body began to tremble violently. After a moment...

"H-Hauuuuuuuu♡♡♡♡♡♡♡♡♡♡♡♡♡♡♡♡♡♡♡♡♡♡♡♡♡♡♡♡♡♡♡ Nnkyuuuuuuuuuuu♡♡♡♡♡♡♡♡♡♡♡♡♡♡♡♡♡♡♡♡♡♡♡!"

※We were only touching fingertips.

"Yura-chan?! Yura-chaaaaaaaan!!"

Suddenly, Yura-chan was screaming and shaking with hearts in her eyes. Then, as if she’d lost consciousness, she collapsed onto the floor.

Fortunately, the nurse's office was nearby, and the nurse was still there. She heard the scream and came running immediately.

Faced with a situation I’d never imagined, I panicked, fled the scene, and hid behind a bookshelf. I should have helped her right then, but back then, I couldn't do anything. I just stood there in a daze, unable to comprehend the catastrophe I’d caused, listening to the fading siren of the ambulance.

Yura-chan was gone the next day. The teacher said she’d transferred schools suddenly, but I didn't know the truth.

I was terrified. I was scared that she’d died just like that, that I’d killed her. But I had no way of knowing if Yura-chan was even alive. When I searched for her on social media, only other people came up.

If it ever got out that Yura-chan ended up like that because of my EXP, what would happen to me? I was scared to death.

From that day on, I hid my arm with an arm cover. I showed my persistent friends only the <None> character to get them off my back. And up until today, I’ve been careful to avoid touching women.

But today, even that finally came to an end.

"I hope Sensei doesn't remember anything..."

I didn't know how much EXP had been transferred to her, but it was clear she’d felt pleasure so intense she’d blacked out. Well, at the very least, she had a pulse and was breathing steadily when I left, so her life shouldn't be in danger.

"I wish this EXP would just go away, even just a little at a time."

Despite my wish, the latest research had confirmed that EXP recovers after a single night's rest. By tomorrow morning, it would be back to a full Muryoutaisuu.

"There’s no way I can ever have sex like this..."

If this is what happens from just skin contact, I’d surely kill any woman I actually did it with.

"I hope you get to fulfill your purpose someday, my son."

I looked down at the "little guy" swaying between my legs in the bath and sighed as I muttered to myself.

The next morning, as soon as I arrived at school, Sakakibara came over to talk to me.

"Mura! Did you hear the rumor about Sagaracche-sensei?"

My heart skipped a beat at the mention of her name.

"Uh, no... What happened?"

"Apparently, she’s quitting being a teacher to go back to being a dungeon diver!"

"Huh?"

That was completely unexpected. For a moment, I breathed a sigh of relief that it wasn't news about her dying.

"That’s so sudden."

"Some people are saying her level shot up and she’s gotten crazy strong! They say your level only jumps like that after you retire if... well, you know. Sagaracche-sensei’s been busy, huh?"

"Hey, don't go making vulgar assumptions."

There was only one way to raise your level after retiring as a diver—in other words, to gain EXP. You had to suck it out of a man.

And that method was "sexual physical contact." Or to use Sagara-sensei’s words from yesterday, "intense sexual acts involving mucosal contact."

It felt a bit hypocritical to think this while scolding Sakakibara, but Sagara-sensei was a beauty with a great body, so maybe she’d been getting busy with lots of guys every day to gain EXP. Otherwise, she wouldn't have gained enough to return to being a diver. Unless she’d touched someone with a truly insane amount of EXP.

"...Ah."

"What’s up?"

"Oh, uh. Nothing."

No, no, no. That "massive amount of EXP" was almost certainly my fault. It must have flooded into her when she grabbed my arm and we fell onto the bed yesterday.

Well, in any case, I was just glad Sagara-sensei was okay. And since she was apparently able to come to school today after what happened yesterday, maybe just touching hands wasn't that big a deal after all. Yura-chan must have been fine too.

Relieved that Sagara-sensei was safe, I spent the day in a bit of a daze until after school.

"Mura, are you here?"

Just after the homeroom ended, Sagara-sensei showed up at the classroom. Since she was the talk of the school right now, every eye in the room turned toward me.

"Er, right. The apology letter for yesterday, right? I have it here."

Even though I didn't have any such thing, I waved a random folded piece of paper as I answered. The class's collective gaze dispersed. I didn't want any weird rumors starting.

"R-Right. Follow me to the nurse's office."

Whether she understood my intention or not, Sensei played along and started walking.

"Hey, Mura, what’s this about an apology letter?"

"You're the one everyone’s talking about right now, Sensei. What if a weird rumor spreads to me? It’s an insurance policy. Now they'll just think 'Mura probably screwed something up again.'"

"...Sorry. That was reckless of me."

"As long as you understand."

We whispered as we walked down the hallway to the nurse's office.

"Get in."

"Got it."

I was ushered into the office and sat down.

Click, clack.

"Click? Clack?"

A strange sound echoed, and the lights in the room went out. The setting sun shining through the window dyed the white infirmary in shades of orange.

"Uh... Sensei?"

I turned toward the entrance. Sensei, who had her hand on the light switch, slowly turned toward me. Her profile was illuminated by the sunset, giving off a red, bewitching aura.

"Now, no help will come."

"What do you mean by that?"

"I said it in every single Health class, didn't I? 'You boys need to keep in mind that you are the prey. Never let your guard down, even with someone you trust.'"

"...Sensei?"

The usually dignified and cool Sagara-sensei was looking at me with feverish eyes.

"It’s your fault for not listening to my lecture, Mura. Oh, Mura, it’s all your fault. It’s your fault for not paying attention to me. And so, bad students need to be punished."

With moist eyes, Sagara-sensei whispered as if trying to convince herself.

"Sensei. I have zero EXP, remember? I don't need to be careful."

"Heh... Don't play dumb, Mura."

Step by step, Sensei drew closer.

"It has to be you. Yesterday, you came to this office, and before I knew it, I’d lost consciousness. When I woke up, I’d leveled up an unbelievable amount. I can't think of any other way I could have gotten that EXP but from you."

"Sensei, you just fell asleep suddenly, so I went home on my own. Maybe someone did something weird to you while you were out? Like a perverted prank. Maybe that’s where the EXP came from."

"I’m a virgin. There was no discomfort in my crotch when I woke up."

This beauty of a teacher was suddenly dropping some serious TMI.

"That <None> character on your arm. It’s a tattoo sticker or something, isn't it? There are guys like that—people who put fake stickers for millions or billions of EXP on their bodies to trick women. Though, I’ve never heard of anyone faking a low number. But after yesterday’s level up, it all makes sense. Mura. You have a massive amount of EXP, don't you? And you're hiding it with that sticker."

"..."

She wasn't right, but she wasn't far off either.

"Please, Mura. Do it with me."

"Do you even know what you're saying? You're talking about laying a hand on a student."

"I don't care if I get fired. I'm going back to being a dungeon diver. That’s a place full of rough characters anyway. Having a criminal record is just an accessory there."

"I'm not talking about that. I'm asking if a serious, righteous person like you, Sensei, is okay with doing something that would hurt a helpless student."

At my words, Sensei's face contorted.

"That’s..."

"You hate unfairness, Sensei. You taught us so passionately so we wouldn't end up getting hurt. And yet, you, of all people, are going to be the one to make a student suffer? I don't want you to do that..."

Appealing to her emotions worked better than expected; Sagara-sensei hung her head.

"...I'm sorry. I was getting carried away. I'm an idiot. I'm a failure as a teacher."

Sagara-sensei began to speak in short bursts.

"Dungeons were my dream since I was a child. Standing on the front lines with my comrades, carving out a new world. That was my dream. Well, it was crushed pretty early on, though."

Sensei gripped her left shoulder—the one where the rest of her arm was missing.

"When I woke up yesterday, my body was overflowing with vitality. I checked my level just in case, and... well. It had gone up an unbelievable amount. I couldn't believe my eyes. And my heart started pounding. If I could get that same amount of EXP one more time, I could go back to the front lines. Even with one arm, I could become strong enough to stand shoulder-to-shoulder with the other divers. I could stand on the front lines again with the comrades who left me behind."

Sensei wiped her eyes as tears began to flow.

"Mura. I'm sorry. I did something stupid. Putting my hands on a student for the sake of my own dream... it’s unforgivable. Before being a teacher, before being a diver, I'm a failure as a human being. Well, whatever. The reason doesn't matter, but the fact that my level went up doesn't change. I'll just work hard from the bottom again as a dungeon diver. Mura, you can go ho—"

”Sensei."

I cut her off and stood up. I took a step closer to her.

"You really are a good sensei. I respect you, so much."

"Stop it. Don't respect a pathetic sensei like me."

"I won't stop. And I don't want the Sensei I respect to die."

If Sensei was going back to being a dungeon diver, it was definitely because of me. And dungeon diving is a profession where your life is constantly on the line.

"Sensei, I want you to be strong. So you don't die easily. So you can fight even with one arm. So you can live and come back to the surface properly."

"Mura, you..."

"Sensei. This is the last time. and you have to keep this a total secret from everyone else."

"M-Mura..."

As I moved close enough to touch her, I could see her moist eyes wavering.

Sagara-sensei was a good teacher who faced her students with sincerity. I didn't want a teacher like that to carry the guilt of "laying a hand on a student."

So, if someone was going to make a move, it should be me.

"Sensei. Try not to die from the pleasure, okay?"

"Mura... Ah..."

Before she could say anything else, I took my hand out of my pocket and touched her cheek.

Her body gave a violent jolt.

"Ah, Mura... nnnnnngh♡♡♡♡ Mu-raaaaaah♡♡♡♡♡♡ A-Ah, ah, ah, ah♡♡♡♡♡♡ Nnkkuhhh!! I... I'm gonna lose!♡♡ I can't hold out!♡♡♡♡♡♡"

※I was only cupping her cheek with my hand.

”Senseiiii!! Hold on! Stay with me, Sensei iii!!"

"I can't... I can't take it anymore!♡♡♡♡♡♡♡♡ My limit... I'm over the line! Aaaaaaaah♡♡♡♡ Nnnnkyuuuuuuuuuuuuuuu!!♡♡♡♡♡♡♡♡♡♡♡♡♡♡♡♡"

"Se-Senseiii!! Hang in there! You can do it, Sensei!!"

It would be a little while longer before the birth of the dungeon diver known as the "One-armed Berserker."