Ishiyama High is a delinquent high school where continual violence and anarchy are the norms. However, there is one unwritten rule: don't mess with Tatsumi Oga, Ishiyama's most ruthless warrior. Oga is walking along a riverbed when he comes across a man drifting down the river. The man splits down the middle after being rescued by Oga, revealing a baby who crawls onto Oga's back and creates an instant bond with him. Although he is unaware of it, this infant is named Kaiser de Emperana Beelzebub IV, or "Baby Beel" for short—the Demon Lord's son! Oga is also faced by Hildegard, Beel's demon maid, as if locating the future Lord of the Underworld wasn't difficult enough. They try to raise Baby Beel together, despite the fact that they are surrounded by juvenile delinquents and demonic powers.