Tōhōshinki Arc

The first story arc in the Beelzebub series. The Tōhōshinki arc consists of Chapters 1-40 in the Manga and Episodes 1-22 in the Anime, and introduces the main characters and several other supporting characters. The arc also details Tatsumi Oga becoming the adoptive father of Beelzebub IV, the son of the Great Demon Lord sent to the Human world in order to destroy humanity.

Synopsis
Spoiler warning: Plot and/or ending details follow.