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.

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.

                                                                  -inspired by : nursyamimi :)-