Ishiyama High is an institution filled entirely with delinquents, where constant violence and a lack of law are standard. Despite this chaos, there is one rule everyone respects: do not provoke first-year student Tatsumi Oga, known as the school’s fiercest fighter. One day, while resting by a river, Oga finds a man drifting downstream. After pulling the man from the water, he splits open to reveal an infant who crawls onto Oga’s back and instantly bonds with him. Unaware of the truth, Oga has picked up Kaiser de Emperana Beelzebub IV, or "Baby Beel," the son of the Demon Lord. On top of being tasked with raising the future ruler of the underworld, Oga must deal with Hildegard, Beel’s demonic maid. Together, they try to care for the infant, but amidst a backdrop of violent juveniles and supernatural abilities, the job proves far more difficult than they expected.