Ishiyama High is a place overflowing with miscreants and malfeasance, where violent outbursts are incessant and disorderliness runs rampant. However, there exists an unwritten code that all abide by—that is, to never cross Tatsumi Oga, a first-year student who reigns as Ishiyama's most savagely skilled fighter. One day, while idling by a riverbed, Oga stumbles upon a man drifting downstream. With his rescue, the man's body surrenders, dividing at the center and revealing a baby who immediately latches onto Oga's back. Unbeknownst to him, this infant is Kaiser de Emperana Beelzebub IV, or "Baby Beel" for short, the spawn of the Demon Lord! As if securing the future ruler of Hades is not enough, Oga is further challenged when confronted by Hildegard, Baby Beel's demon maid. Together, they strive to raise the infant demon. Yet, with adolescent hoodlums and otherworldly powers threatening their every move, Oga and Hildegard may have bitten off more than they can chew.