Beelzebub's Tales uses the framing device of the musings of an extraterrestrial known as Beelzebub (who shares a name with the demon of the same name, to his grandson Hassein, as they travel through space towards Beelzebub's home planet, Karatas, on the spaceship Karnak. Beelzebub recounts his adventures and travails amongst the "three-brained beings" (humans) of the planet Earth. Beelzebub details the history, custom and psychology of humanity and relates the esoteric theory behind the workings of the universe.