Ishiyama High—land of the delinquents. No matter how tough you think you are, you don't mess with an Ishi-high student. One of the most infamous students is first year Tatsumi Oga, called "Demon King" by those he's defeated.

One day, while Oga is finishing off a gang that attacked him, he stumbles upon an infant. In a rare moment of kindness, Oga tries to care for the baby. As a result, the child becomes overly attached to him. In a panic, he brings the baby to his best friend Takayuki Furuichi's house. While there, they are attacked by Hildegard, a demon maid who says the baby is actually Kaiser de Emperana Beelzebub IV. She also reveals that he was sent here to destroy humanity, on a whim of his father Beelzebub III, Great Demon Lord of the Demon World. After failing to remove Baby Beel from Oga, she declares that he must raise him, as moving more than 15 meters away will result in instant death for Oga.

With his delinquent past and unfriendly nature, Oga must deal with the burden of raising a demon baby. Beezelbub follows his story through encounters with demons and dangerous classmates, all while being an unorthodox role model and carer for the young Baby Beel.