Tatsumi Oga, a delinquent student at the worst high school in Japan, arrives at school with a small baby who emits massive shocks of electricity whenever it cries. Tatsumi claims he found after splitting open a strange man who came floating along the river, though his friend Takayuki Furuichi does not quite believe. While at Takayuki's house, they are both approached by a demon woman named Hildegarde who claims the baby, known as Kaiser de Emperana Beelzebub IV, is the son of Devil King who was sent to Earth to find a human to raise him so he could destroy humanity. When attempts to retrieve Beelzebub fails, Hildegarde notes that Tatsumi has been chosen for the job. When Tatsumi refuses, Hildegarde attempts to kill him with a bird demon but Tatsumi manages to beat it. When he is cornered by Hildegarde and a bunch of thugs and his cheek is cut, Beelzebub goes into a tantrum that only Tatsumi is able to stop. When a damaged pylon nearly falls on Beelzebub, Tatsumi steps in to save him and resonates with Beelzebub's power, allowing him to disintegrate the pylon. After Tatsumi recovers from the ordeal, Hildegarde decides to stay with him to raise Beelzebub, much to Tatsumi's chagrin.
—Wikipedia