When I call this story rational, it means in the sense that the characters act reasonably and think cleverly. It doesn't mean that everyone is a perfectly logical thinker, just that they don't do things which in the original work seem to be for more rule of cool than something which makes much sense story-wise.
Also, I haven't read the full manga, and am being guided by the wiki for more details on canon events and the powers of the characters.
Chapter 1: Demonic Discovery
"Come back safe!" The Kamido family called out to the eldest son of the family, Tanjiro. He was heading to town to sell charcoal.
"I will!" he called back and breathed in deep, letting the crisp winter air fill his lungs. His boots crunched as he walked through the fresh snow on the path. People would definitely be eager for charcoal on a day like this. Maybe he could even use some money to buy something nice to bring home.
His time in the town ended up being eventful and enjoyable. Tanjiro used his sharp nose to settle a dispute about a broken vase, it turned out that a wily cat had knocked it over, and did odd jobs for people who asked. He never went hungry when he went into the town by his house, the generous townspeople offered him food and hot tea.
He even managed to sell all his charcoal, although it had taken the whole day. It had gotten dark, and his family would probably be asleep by the time he got back.
Along the way, old man Saburo called out to him. "Hey, Tanjiro!"
"Yes, Mr. Saburo?"
"Where are you going? Don't you know it's dangerous being out at night?"
"Ah, it's alright. I know to avoid any animals, and I remember the way home well enough to make my way there even in the dark."
The old man scoffed. "There are more than just animals lurking out there. Come over here!"
When Tanjiro entered, he noticed that the old man had flowers all around his house. It was . . . an interesting choice of design for a solitary man in his fifties. "You have a very pretty home, sir."
"I don't give a damn how pretty it is. These wisteria flowers are to ward off demons."
"Really?" Tanjiro asked curiously. "I heard wisteria gives good luck, but I never heard they protected you from demons too."
"Luck is superstitious nonsense. No, the real power of the wisteria is in repelling demons. I've always had these flowers in my home, and I've never been attacked by a demon!" Mr. Saburo proclaimed proudly.
Tanjiro didn't think that logic was quite right, but he didn't argue. Mr. Saburo probably didn't get a lot of company, and the boy didn't mind taking shelter from the cold here.
Mr. Saburo turned out to be a fairly good cook as well, providing Tanjiro with a tasty dinner of rice, fish, and a vegetable soup before he went to sleep. As the boy drifted off, he recalled his grandmother. She used to tell him stories about scary demons too, and had always liked to pin wisteria flowers on her clothing . . .
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"Thank you for letting me stay the night." Tanjiro bowed to Mr. Saburo the next morning.
"As if I could let someone wander around the forest in the middle of the night. Be more careful and I won't have to bother."
"I will, sir."
"Wait!" Mr. Saburo said just as Tanjiro was about to walk home. "Before you leave, take this for your family." The old man handed him a vase full of wisteria flowers.
"Ah . . . don't you need these to help protect you from demons?"
"I have plenty to spare."
Tanjiro accepted the vase and made his way back home. Mom, Nezuko, and Hanako would probably appreciate the flowers.
As he approached his house, something strange started to happen. The scent of wisteria became much stronger. At first he looked at the vase, wondering whether something was happening to the flowers inside. Then, he realized that there was more wisteria somewhere nearby. A lot of it. And strangely enough, it also seemed like somebody burning incense made of the stuff too.
Another scent joined the surge of wisteria. Blood. Blood and unfamiliar people. Tanjiro quickened his pace, rushing to his house. His legs fought through the snow, kicking it up behind him.
Could it be bandits? Were his family okay? What was going on?
His house came into view, and it was surrounded by dozens of people. Most of them wore black uniforms, their heads covered by black cloth, with only their eyes visible. They were heavily scented with wisteria, and it looked like they'd placed pots around his house that were burning it as incense. Some of them wore packs, and others were calmly walking in and out of his house. Only a few people had their faces visible: two young men, a boy, and a woman wearing the same kind of black uniform.
Tanjiro glanced at them only briefly, his eyes shooting over to the stretchers on the ground covered in bloodstained cloth. Their shapes were horrifying hints of what lay underneath.
"Hey! Hey!" he shouted at them as he ran. "Who are you people? What are you doing at my house?"
A few of the masked people blocked his way, their motions strangely fluid and graceful. "Stop!" One of the masked men called out in a clipped voice.
Tanjiro stopped, but his mind kept racing through questions. Were these people shinobi? Why were they here? Most importantly, where was his family?
He avoided looking at the stretchers.
The four unmasked people had wandered over, swords at their hips or back. The woman was actually a young girl, not that much older than him. She had a butterfly pin in her hair and a butterfly patterned cloak. In a soft voice, she asked, "Do you live here?"
"Y-yes. I do, I live here with my mom and five siblings. Where are they?" Tanjiro forced himself to count the stretchers. There were only four.
She gave him a sad look. "I'm sorry, young man, but they're dead."
"No no no no, they can't. They can't!"
"They are." The boy said in a monotone voice. "Would you like to see the bodies to confirm their deaths?" He looked even younger than Tanjiro. There wasn't the slightest hint of caring or empathy in his voice, at best the only emotion was a trace of impatience.
"How could you even think to say something like that?!" The girl admonished him and then turned to another young man beside her. "Tomioka, can you please take Tokito away?"
Tomioka did so, leaving the girl with the man who was bedecked in jewelry and armed with two swords on his back. The girl gave Tanjiro an apologetic look. "Please forgive his insensitivity. What is your name?"
"Kamido Tanjiro," he said in a distracted voice. Four stretchers. Four not six, four. That meant they weren't all dead.
"Kamido, we found the bodies of an adult woman, three boys, and two girls. If that matches the description of your family, then I'm afraid to tell you that they've passed."
Just as she was finished speaking, there was a commotion from inside the house.
"She's alive!"
"Wa-wait, what's she doing?!"
"Fools, she's a demon!" Tomioka and Tokito sped into the house in a flash. The girl followed them in a similar burst of unbelievable speed.
"Restrain her, don't kill her yet!" the girl's voice sounded from inside the house.
Tanjiro ran in after them. Had he heard her right? He entered the house in time to see Nezuko, covered in blood and held in place by Tomioka. The girl and Tokito were holding their swords, pointing them at each other.
"Whoa there." The bejeweled man restrained him from behind. He was a large, well-muscled man, but he seemed even stronger than he looked.
"Let me go! I won't let you people kill her!" Tanjiro struggled and tried to break free, but it was no use.
"Kocho, why are you stopping me from killing her?" Tokito asked.
"This young man doesn't understand what's happening," the girl replied. "At the very least, we should explain the situation before we do our duties."
"For what it's worth, I agree," the bejeweled man said. "Kid just learned his family's dead, and now he's seeing that his sister's still kicking. We could at least ease him into the truth."
"We also don't want civilians to resent us and obstruct us in our duties," Tomioka added.
At that, Tokito lowered his sword. "I suppose, although I don't see how one boy could cause much trouble for the Corps."
Kocho sheathed her strangely shaped sword and faced Tanjiro. "Kamido, it seems we weren't completely accurate in what we just told you. Your sister has survived, but she's become a demon. A crazed, bloodthirsty monster who eats human flesh."
Tanjiro shook his head. "What are you talking about?! You're crazy!"
"Look at her," Kocho said. "See her struggling. She's no longer your sister. Right now she's little more than a starving animal that can only be satisfied by consuming humans."
Tanjiro looked to his sister. "Nezuko, tell her that's all nonsense. You're my sister, right?"
"Raagh!" Nezuko roared at him, spittle flying from her lips. Her teeth had become dangerously sharp, and there was no recognition in her wild eyes.
Not only that, but something else was happening. Her body was growing, she was getting taller and her muscles more prominent. The nails on her fingers grew and sharpened to become more like claws. She stomped on the floor, breaking the wooden boards, and struggled harder. Still, Tomioka held her as firmly as a statue.
"Do you recognize the truth now?" Tomioka said. "This is a Demon Blood Art. She's not human, not your sister. Not anymore." She was shrinking again to her normal size, and her struggles died down. The effort seeming to have tired her. He began taking out his sword.
"Wait! She's still my sister! Even if something's wrong with her now, there could be a cure, right?"
Kocho sighed. "There might. But please, don't hold out hope for one. She's a danger to other people, and we can't spare the resources to restrain her for long."
"Then I'll do it! I'll keep her away from other people until a cure is found!"
The bejeweled man laughed. "Ha! That's a flashy thing to say. I'd almost be interested in seeing you try."
"You still don't understand," Tomioka said. "Even if you're her brother, she won't hesitate to eat you. Mother or father, husband or wife, son or daughter. Such human bonds mean nothing to them." He paused. "Then again, these are all just words. Perhaps you need to see the truth with your own eyes to accept it." He turned to one of the masked people. "Bring in one of the bodies."
Kocho argued, "Tomioka, what are you-
"We're about to kill what used to be his sister. This isn't going to make things much worse."
"Wh-what's going on? What are you doing?" Tanjiro asked.
"You'll see for yourself the inhuman nature of demonkind," Tomioka said.
A stretcher was brought over, and Nezuko immediately reacted. She sniffed the air and her mouth began watering. Her eyes were fixed on the covered body.
"Take off the sheet," Tomioka said.
The body was revealed, and Tanjiro teared up at the sight of Rokuta's face. His youngest brother's eyes were shut, and his chest had been caved in. Red stained his robes, and his skin was now pale instead of the lively pink it had been just yesterday.
Tomioka shoved Nezuko at Rokuta, and she picked up even more speed by lunging at the body.
"No, Nezuko," Tanjiro begged as he weeped. "No. That's Rokuta. Rokuta!"
Nezuko paused. Slowly, through Tanjiro's blurry eyes, he saw her face him. In a low voice she said, "Big brother?"
Then she growled and leaped at him.
"Shit!" the bejewled man said. He threw Tanjiro to the side.
When Tanjiro landed a few feet away, everyone was silent except for Nezuko. She growled over him. He wiped his eyes and looked up. Nezuko stood over him protectively, glaring fiercely with her teeth bared at everyone else. They had their swords drawn and watched with astonished faces.
Tanjiro got to his feet and pulled her in a tight hug. "Nezuko. Nezuko. Oh thank God you're safe."
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After Tanjiro had explained that the people who called themselves the Demon Slayers weren't going to hurt them, Nezuko had calmed down and seemed to go to sleep. It wasn't like sleep when she'd been a human though, it was far harder to rouse her awake than a human. Shaking her or loud noises hadn't woken her.
The Demon Slayers still seemed to be wary of her, but they took the time to explain what had happened. His family had been attacked by a demon. Not just any demon, but the Demon King Muzan. He had killed the rest of the Kamido family and turned Nezuko into a demon.
One odd thing though was that his mother's head was missing. None of the other bodies had body parts missing, they'd been killed with blunt force or lacerations.
"I know you probably don't want to hear about your mother's corpse being desecrated," Kocho said. "But we're on an important mission to hunt down Muzan. One clue is that he didn't take the time to eat any of the bodies. His killing was quick and efficient, except in the case of removing your mother's head. We don't actually think he 'ate' it, at least in the usual sense of the word. We think he absorbed it. Can you tell us any reason he might do that?"
Tanjiro shook his head. "Until today, I didn't even know demons existed. I don't know why he would do that." He looked around. "Do you think it would be a good idea to use wisteria to protect Nezuko and myself from him? Nezuko will probably hate it, but I can't think of any other way to stop him if he comes back."
"Truthfully, all this wisteria is more wishful thinking than anything else," Kocho said. "It works great against weaker demons, but we've noticed a reduced effect against more powerful ones. For Muzan himself, wisteria might only be a minor irritant."
"Oh."
"Curing your sister will require more than just keeping watch over her. Would you like to join Demon Slayer Corps?"
"Can I?"
"Of course!" she nodded. "We're always eager for recruits."
"Really? How come I've never heard of them?"
"For a number of reasons, the Corps decided that it's better to keep the existence of demons a secret from society at large."
"Okay." Tanjiro hesitated. "Then I want to be a Demon Slayer."
"Are you sure? It's not a job for anyone, and hunting Muzan himself for a cure for your sister is going to be immensely difficult. To tell you the truth, this might be the closest we've ever come to catching him."
"It doesn't matter how difficult it is. I'm still going to try."
She beamed at him. "Great! Welcome aboard!"