The delinquents of Ishiyama High School either fear Tatsumi Oga or wish to defeat him, but the young fighter finds himself preoccupied with other matters—like tending to baby Kaiser de Emperana "Beel" Beelzebub IV, the son of the Demon Lord.

Baby Beel's maid, Hildegard "Hilda," informs Oga that the infant has latched onto him due to the former's immense strength. In turn, Oga must take responsibility and raise the Demon Lord's son, or face Hilda's blade—that is, unless he can find someone stronger than him to appeal to Beel. Perhaps Oga can convince one of the Tohoshinki, composed of the four strongest fighters of Ishiyama High, to take Beel off his hands!