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The story follows Tatsumi Oga, a first year student at a school for juvenile delinquents called Ishiyama High. The story starts with Oga telling his best friend, Takayuki Furuichi, the strange story of how he found a baby. One day while doing 'laundry' by the river, Oga saw a man floating downstream. Oga pulled him to shore but the man split in half, revealing a baby boy inside. This baby turned out to be the son of the great demon king, and Oga has been chosen as the one to raise him, along with the baby's maid, Hilda. The manga follows Oga's life as he tries to raise the child while enrolled at Ishiyama High.
In the early chapters, he attempts to 'pass on' Beel to other students at Ishiyama, thinking that if he can find someone stronger and more evil than himself, the baby will attach himself to that person instead. He decides to try to pass the baby on to the Touhoushinki, Ishiyama High's strongest four students. Oga meets and fights Hajime Kanzaki, the first member of the Touhoushinki and the one who is closest to dominating the school; however, Oga defeats Kanzaki, which causes Beel to find him even more appealing.
Later, Oga discovers a peculiar symbol etched on the back of his right hand. Hilda explains that it is the fly king's seal, more commonly known as the Zebub spell, and serves as the family crest of the royal family. The appearance of the seal means that he has officially made a contract with Beel. She also explains that the more evil the bearer of the seal is, the more complex the design of the seal will become. Oga attempts to prevent the seal from growing by avoiding fights with other students, but gives up when Himekawa, the second member of the Touhoushinki, captures Hilda and Furuichi. Oga infiltrates Himekawa's base to rescue them and defeats him by using the Zebub blast for the first time. The consequences of using the seal's power cause the crest to stretch across his arm. Hilda comments after looking at the crest that the amount of energy it used would have driven a normal human mad.
While visiting the park, Oga meets a girl named Aoi and her brother, Kouta. Oga asks her immediately to go out with him, which causes a misunderstanding that stays within Aoi's heart. Oga tells Beel to fight Kouta, possibly as a joke, but Kouta accepts the challenge and easily pushes Beel off a bench. This event sparks a rivalry between the two babies. It is later revealed that Aoi, who was in disguise the whole time, is in fact Aoi Kunieda, the leader of the Red Tails and another member of the Touhoushinki.
Aoi, who was away from school during the earlier events of the manga, has returned to Ishiyama High to defeat Oga in order to prevent him from causing any more trouble within the school. When they encounter each other in battle, she realizes that she has already met Oga in the park, but he doesn't recognize her. After he dodges her attacks twice, he notices Beel's interest in Aoi and asks her to become Beel's mother. Although he means for her to replace him as a parent, his request causes another misunderstanding similar to the one in the park. The fight is postponed, and Hilda soon arrives to give Beel his milk. The MK5 make their debut in the story, only to be defeated in a five panel pattern which appears again later in the story. After the fight, Beel senses something which turns out to be the last of the Toushinki, Toujou. Oga, however, does not notice.
Aoi sees Oga and Hilda's fight with the MK5, and believes the rumors that they are husband and wife. She is challenged by Hilda to a fight so she can test her ability, but the fight ends quickly with Hilda deciding that Aoi isn't on the same level as Oga. At the same time, Oga is confronted by Chiaki and Nene. Two female students tell Aoi that Oga injured the two of them, which later turns out to be MK5.
Aoi fights Oga to avenge her two classmates, while Oga assumes that she wants to accept the role of becoming Beel's parent. At the same time, Nene wakes up in the infirmary and goes to warn Aoi of MK5's trap, which turns out to be Miwa's scheme. MK5, however, prevents anyone from interrupting Aoi and Oga's fight. Nene is about to get shot, but Natsume appears and fights with MK5 to allow Nene to tell Aoi. She was unfortunately too late, as Aoi seems to have defeated Oga, and she comes just in time for Miwa and Ikari to take over.
Oga suddenly gets back up and defeats Ikari and Miwa with ease. It is then shown that Furuichi and Hilda have been watching the whole time. Oga, Furuichi, Hilda and baby Beel go to the beach in search of Toujou to hand baby Beel over to him, not knowing that this is a plot set up by Kanzaki and Himekawa, who want Toujou and Oga to crush each other. Aoi goes to the beach as well to warn Oga of Toujou's strength. Oga and the gang have troubles searching for Toujou and are sent on a wild goose chase. Oga decides to write a letter of challenge to Toujou and have Aoi pass it to him. Aoi attempts to tear it up, but one of Toujou's henchmen finds it. They are supposed to meet between the two rocks at 5 PM. Oga is confronted by the MK5 once again, but quickly defeats them and arrives late. Toujou has left after crushing several motorcycle gangs, which inspires Oga to find Toujou. Later, baby Beel starts feeling sick and it appears that he's come down with a cold. However, Oga's tattoo mysteriously vanishes, and later that night Beel disappears. Hilda kicks Oga out of the house and leaves to find help in the demon world. Oga goes off to find Beel, but runs into Tojou, who, as it turns out, has Beel on his back. After a scuffle, Toujou comes out as the victor, and asks Oga to challenge him again sometime. Once Oga gets home, he is in a surprise to find Hilda there waiting for him, with a strange demon doctor known as Dr. Furucas, and his assistant, Lamia. Furucas reveals that the cold is actually Beel's way of coping with a rise of his power, by keeping the energy within himself, and that he cut off the link between him and Oga to protect him. Lamia then injects Oga with a drug that allows him to reestablish the connection with Beel. Oga then goes to confront Tojou with the aid of the three other Tohoshinki. Once he makes it to Toujou, Beel returns to normal and goes back to Oga. Toujou and Oga then have one final duel in which Oga is victorious, but baby Beel's influx of power causes him to destroy Ishiyama.
Due to their school being destroyed, the students of Ishiyama can only go to their sister school, Saint Ishiyama. Initially, they are constantly looked down on, but after a volleyball match against the Six Knights, the negative views towards them stop. During the volleyball match, Oga's demonic power is displayed due to a fight with someone who had a grudge on him, though it is covered up by the Saint Ishiyama president who claimed that it is just a trick. The president turned up to be a demon himself.
A new teacher soon arrives to replace the one teaching the Ishiyama group, knocking out both Oga and Toujou on the same day before carrying them to the classroom. The teacher, Zenjiro Saotome, later appears when Oga, Hilda and Aoi are attacked by Hecadoth, and puts up a fight against the demon. Later, Naga and Graphel, the other two demons who came to the world in search of Oga, appear before Saotome, but Saotome uses his Spellmaster spell to attack them. After realizing that he is a Spellmaster who is too powerful for them to win against, Naga uses a teleporting jewel to take the three demons back to Demon World. After that Saotome, the spellmaster althogh having Oga refuse he ends up training him.Source: [You must be registered and logged in to see this link.]