Kazuki Muto, a student at the Ginsei Private School, one day decided to check out an abandoned factory that was in the forest behind his school. As he entered the factory, often called the Monster, he heard a noise, and turned to see a giant snake-like creature.
Horrified, he turned, and ran, hiding behind a wall, and looked carefully around the corner. As he peered around the corner, he saw a girl standing there, and she seemed oblivious to the giant, evil-looking cobra like creature. Throwing caution to the wind, Kazuki ran forward, pushing the girl out of the way as the creature struck. He felt a horrible pain in his chest as the attack struck him in the heart.
Kazuki woke with a scream, causing all of the people living in his dorms to charge into his room and asking him why he is screaming. Kazuki, who is still asleep, screams that he has died, before he began attacking his friends with karate, awaking both the boys and girls dormitory with the noise.
The next day, Kazuki went to school, where two girls commented on the events last night, to which Kazuki flinched back in shock, asking how news of his sleep-induced assault had spread so quickly, only for one of the three he had attacked, Hideyuki Okakura, to appear behind him, telling him that he had told all of the students. He then lifted up his shirt, displaying multiple bandages for everyone to see, saying how those injuries would have given it away even if he hadn’t told about the attack. Kazuki denied this was him, saying that if it was the karate he had learned it would have killed Okakura, before swiftly turning and striking a pose, saying how he instilled terror in everyone, much to the chagrin of Okakura. Okakura then told Kazuki to apologize, a command which was quickly met, and the matter was set aside between the two, just then, the other two who had been attacked came up behind Kazuki. One of them, Daihama, mentioned how the sleeping Kazuki was terrifying. The other one, Rokumasu, asked Kazuki what he the dream had been about. The friends began walking, and told them about how he had gone to the monster factory, how he had seen the girl, and how he had died saving her. The fact that he didn’t kill the dream monster shocked his friends, but this shock was quickly ignored and they continued on their way to class.
As the four students were talking, the school bell rang, and they made a mad dash for the gate, where one of their teachers, Mister Mita, was standing. Kazuki’s friends got into the gate, and Rokumasu turned, telling Kazuki that he was running too slowly. Kazuki just smiled at this comment, before striking a pose and saying how he was about to reveal his amazing dash. As he prepared to run, his sister, Mahiro, came running up, and he ran along side her, congratulating her on being late on her first day. She responded by telling him that they could still make it into the school on time, and Kazuki playfully pushed her forward. Mahiro crossed the border into the school just as the late bell rang, making her on time, but Kazuki was not so lucky, being one step away from the border. After the run, Mahiro leaned on the gate, but as she leaned, it suddenly began closing, since school had started, and she fell into the way of the closing gate. Quickly, Kazuki went to his knees, putting a hand against the automatically closing gate as he held it opened for her, and, unseen by everyone, a light in the outline of a hexagon appeared on the left side of his chest. He pushed Mahiro out of the way, and the gate continued to close as he quickly fell out of the way, receiving only a few scrapes. As Mahiro bent to see if Kazuki was alright, Mita walked up to the two, and gave both of them a point for being one second late, and reminded them that if they got three points, they would be punished. Kazuki argued that his sister was on time, but Mita insisted that she was late, so Kazuki convinced him to take give him two points, and make it so that Mahiro didn’t have any.
Looking back at the figure of Mita, Okakura commented on how stupid the guy was being, but Rokumasu told him to be quite before he was heard, and the group began to leave as Mita stared after them.
As they were about to get away from Mita, he told all of them to not move, before asking Kazuki why he did not have a school regulated backpack. Kazuki turned back towards the teacher, and told him that he could not find it that morning, so he had grabbed an old one to carry his stuff in. Mita then made a strange comment about where Kazuki went to amuse himself, but he quickly said that it did not matter. He then gave Kazuki one more point, totaling the three he needed to get a punishment. He set the punishment as pulling weeds from the school field after school, and forbid him from going home until he was finished pulling every weed. Kazuki was about to protest the punishment, but Mita stopped him, telling him that the bell was going to ring, and that, unless he wanted more points, he would get to class. Kazuki turned, defeated, and began walking to class. Unbeknownst to him, Mita stared after him as he left, with a strange look in his eye.
Later that day, at lunch, Kazuki, Okakura, Rokumasu, and Daihama discussed the punishment, Okakura noting how Mita had been acting very strange lately. Rokumasu did not agree, saying that he had always been one of the hardest teachers to get along with. As they talked, Mahiro entered the area they were having lunch in, and ran up to the three, Kazuki having gone to get drinks. After learning this from the others, Daihama complimented how she looked in her school uniform, and Kazuki came back to Mahiro doing a little dance. Mahiro then told everyone that the biggest reason she had wanted to go to the school was because of how the uniforms looked. To everyone’s shock, Kazuki then told how he had only wanted to come to the school for the uniforms as well, before turning to his sister and commenting on her good mood. He then decided to give her his drink in honor of her getting into the Ginsei School (unfortunately, he enjoys a drink that everyone hates, and she was forced to drink the horrible tasting liquid.)
Turning to the others, Kazuki told them how the area around his heart had been hurting. He then looked down at his cell phone, noticing that he had a message, but before he could see what the message was, Rokumasu, who hadn’t noticed he had a message, told him that he needed to stop fooling around, or else he would have more problems then a hurting area around the heart. Daihama then agreed, telling Kazuki that something bad could have happened, and when Kazuki mentioned that he had never thought about that, Okakura told him that he should start taking that into consideration. Kazuki justified his actions by saying that his body had moved before he could think, and that it didn’t matter anyways as long as Mahiro was safe. Again, Rokumasu spoke, telling him that, if he wasn’t careful, his dream would become a reality. Kazuki became a bit freaked out at this comment, saying that he never wanted to relive that nightmare. Before anything else could be done, the bell rang, and everyone headed of to class. Before they had gotten out of the courtyard, though, Mahiro turned to Kazuki, asking if he needed any help his punishment, but he denied the offer. Then, as he waved goodbye to Mahiro, he realized that he was still holding his cell phone, and quickly checked the text message that he had received, and became shocked at the message before him: “Please take good care of your new life.”
After school, Kazuki reported to his detention, and began pulling out the weeds in the field. He worked well into the night, but finally finishing, he began moving to report his completion. As he walked, he noted just how creepy the school was at night, and, turning his head away from the school, he saw the monster factory. As he stared at the abandoned factory, the memory of the dream, as well as the strange message he had received flashed through his mind, and he became frightened. Losing his nerve, he turned around, telling himself that he would just report his completion the next day. Before he could leave though, a voice behind him asked him where he was going, and he turned to find Mita.
Mita asked him if he had finished his work with the weeds, but then ignored his own question, saying that they had something more important to attend to, much to the confusion of Kazuki. Mita then through something to Kazuki, and he recognized his school bag. He turned back to Mita, asking him where he had found the bag, only to be told that he had found it at the old factory behind the school. Scales then began forming on Mita’s face, and with a savage look in his eye, he told Kazuki that the bag had been left the previous night by the man who had stopped him from eating his dinner the previous night.
At the shock of this statement, a hexagon-shaped light again began radiating from Kazuki’s chest, and he fell to the knees in pain. Kazuki then told himself that he was only having another dream, or else he would be dead, while at the same time grabbing a pole that lay near him. Before he could do anything with the pole, though, Mita told Kazuki that he hadn’t fully exterminated him the previous night, and that this time, he would make sure that Kazuki was good and dead. Mita’s body then began to shift and stretch, growing to the form of a large, metallic snake, just like the one that Kazuki had seen when he believed he was dreaming.
Kazuki turned and ran from the huge serpentine creature, realizing that he had not been dreaming as the creature began chasing him. As he run, his cell phone began to ring, and he answered it only for a female voice on the other end to tell him that she would be there in three minutes. He then began asking the voice who she were, what was going on, and what was going to happen to him, but before he could be answered, the monster behind him attempted t attack him, and he dodged it. After this, the voice told him that talking would only delay her, so she would keep the explanation brief.
She told Kazuki that the thing behind him was a Homunculus, and that they would take on the guise of humans so that they could devour them, much to Kazuki’s horror. The voice then told him that she had accidentally pulled him into this, but before he could ask her what she meant, he heard the Homunculus catching up to him, and when he told the girl that the thing was catching, up, she told him that there was only one other option, allowing him to use his power to fight the creature. This surprised Kazuki the most out of all that the girl had said, and he asked her what she meant by his power, and if she was serious and that he was really supposed to fight the Homunculus. She then noted that he could not remember what had happened, and when he told her that he was getting out of there, she told him that it was useless, as the Homunculus would follow him to the ends of the earth until he was dead. Kazuki then turned the corner, and saw directly in front of him his sister, Mahiro.
He quickly asked her what she was doing there, and she tells him that she came to help him, since he was taking so long. Before he can do anything else, Kazuki hears a sound, and quickly turns his head to look behind him, and then quickly ran towards the sound, telling Mahiro to run while he distracted it, but as he ran, he heard the sound of ground breaking behind him, and he turned as the snake’s jaws burst through the ground around Mahiro. Its jaws closed before she could react, and its head rose high above Kazuki, and slowly looking down at him. Its mouth then opened and in the back of its throat, a tongue made its way out. On the end of the tongue, Mita’s head rested, and it began to speak, telling Kazuki that Mahiro would make up for the meal he had lost the previous night. Before the head could say anything else, though, Kazuki lunged forward, beating it with the pipe he had picked up earlier. He began shouting that she had had nothing to do with this, and to give her back as tears slowly streamed down his face.
The half-destroyed head quickly looked up at him, telling him that it was useless before it swung forward, hitting Kazuki square in the chest. He flew backwards, hitting a tree as the monster told him that it had been created by alchemy, and therefore could be injured by nothing but alchemy. Hearing this, Kazuki remembered that the strange girl on the phone had told him he would be allowed to use his powers, and the hexagon shape again appeared over his heart. As the pain from the glow filled him, hi mind suddenly had a strange memory with the girl he had saved the preceding night.
Both Kazuki and the girl were on top of the water tower, Kazuki slumped on the ground and looking at the floor, and the girl standing before him. She asked if he could still hear her, stating that he had already lost his heart from being so rash and jumping to save her. Her voice then became less harsh as she reached into her pocket, noting that he had only done it to save her. From her right pocket she then pulled out a metal, hexagon-shaped object, calling it a Kakugane, a special object which had been created and refined by alchemists. She then leaned over, bringing the object to his chest, and it slowly phased into where his heart should be. She told him that he now had the power of a new life, as well as another power, a power that would only awaken through his will to fight. This power would then take the shape of any weapon he wished.
Back in the present time, as Kazuki was still on the ground from the snakes attack, he slowly stood up as the events from the previous night still streamed through his head. Mita’s head then looked down at him, telling him that he had already dead, and then asking him if he wanted to die and join his sister in the afterlife. Kazuki’s hand then became taut, and he bought his hand to his chest, telling the monster in front of him that he would die, as long as the creature gave him back his sister. He then shouted out the word “Buso Renkin.” The Homunculus then ate him, saying that he had no time to bargain though, but it suddenly began shaking, and from the inside its stomach, Kazuki, holding Mahiro, erupted from it, Kazuki holding before him a lance, created from hid replacement heart.
As Kazuki slowly fell towards the ground, he saw before him a girl in a skirt, and she spoke calmly, telling him that the lance was a perfect mirror of his personality, as it never thought as it ripped through its enemy. She was holding a Kakugane at her legs, and she too shouted Buso Renkin, and two leg braces appeared on her thighs. Attached to each of these leg braces were two scythes. She quickly killed the creature, stabbing it through a seal that rested on its head, and then she turned to Kazuki in mid-air. She had a deep scar running from cheek to cheek. As he looked, Kazuki realized that the girl with the scar was the girl he had saved the previous night. The girl looked at Kazuki, and told him to stab the Homunculus through the head in the seal, and to be careful to land, or he would die, and, looking down as he fell, he saw how high up he was.
After Kazuki and the girl had landed, he told her how he had ended up in the factory after seeing the monster go in and seeing the factory lights on. He then stuck a pose again, saying how he had rushed in to save her, as she was obviously going to die, but the girl disagreed, saying that she had only looked defenseless to lure the monster out. He then made a face, saying how he had clearly died. As he mentioned this, Mahiro, who was at that time propped up against a tree, stirred, and Kazuki ran towards her, throwing his lance down in the process. He suddenly felt weak, and fell, only to be caught by one of the scar girl’s scythes. She told him to take it easy, since Mahiro was in better shape then he was, and when he told her that knowing she was ok made him feel better, she called him strange for his unbelievable worry for others. The girls Buso Renkin then began returning to its normal state as a Kakugane, and she told him that what he had was bravery, and a very reckless bravery at that. She then turned to him ad smiled, telling him that she didn’t care about that though, and that she at least liked him a bit, causing Kazuki to blush.
The girl then lifted up Kazuki’s lance, handing it back to him, and telling him to be careful with it, as it was his heart, his new life, and that he would die if it was every destroyed or taken away from him. He then lifted up the lance in front of him, before turning to the girl, and asking her what alchemy. She told him it was in the dictionary, and then began to tell him what alchemy really was as the night went on.