At Ishiyama High, a school infamous for its delinquent students, first-year Tatsumi Oga—nicknamed "Demon King"—rules supreme over those he has defeated. One day, after taking down a gang that attacked him, Oga ends up looking after an abandoned infant. Despite his initial panic, he brings the baby to his best friend, Takayuki Furuichi, where they encounter Hildegard, a demon maid. She reveals that the child is Kaiser de Emperana Beelzebub IV, sent by his father, the Great Demon Lord Beelzebub III, to Earth with a mission to destroy humanity. Hildegard warns Oga that if he strays more than 15 meters from the baby, he will face immediate death.
With his rough demeanor and history of violence, Oga reluctantly becomes the caregiver of the demon infant. The series follows his journey as he deals with both demonic beings and his dangerous peers, all while acting as an unconventional guardian and mentor to Baby Beel.