Fearful of what their recent test results could mean for them at the Royal Academy of Diavolo, the eldest of the seven brothers, Lucifer, sits his brothers down for a much-needed family study group to up their average. However, when his brothers struggle to focus and get any study done, Lucifer must take drastic measures to stop them from slacking off.

Alone in his room, third-born Leviathan lets his inner cosplayer loose as he puts on a one-man show dressed as his favorite character Ruri Hana from "The Magical Ruri Hana: Demon Girl." However, when his younger brothers have little care for boundaries, Leviathan must hide his cosplay as sixth-born Beelzebub and seven-born Belphegor come unexpectedly barging in.

Welcome to the bunny boy café in which the brothers dress up as bunnies! If you find a bunny you like, please give him a carrot. The brothers will do their best serving you to earn a carrot. Please enjoy your time and relax. Oh? Your time is up with the bunny boys! You have one carrot left... Which bunny will get your very last carrot?

The brothers are at the private beach of Lord Diavolo, the only son of the Demon King (Levi isn't much of an outdoor type so he is absent). The ever-so-poor Mammon secretly tries to carry out his plan, which is to take a shirtless photo of Lucifer and sell it to Lucifer's fans at RAD. However, his opponent is a tough one! With no openings, there are no photo opportunities. Mammon has had it and comes up with a final plan. Will he succeed...?

In a desolate land oddly reminiscent of a JRPG, the final boss Beelzebub sits upon a throne with his captive, Princess Asmo, by his side. To save the princess, three adventurers, Leviathan the Hero, Satan the Warrior, and Belphegor the Mage, confront Beelzebub. But when the mighty Leviathan slays Beelzebub with one swing, the adventurers don't receive the reaction they were expecting.

It seems as if Satan has been reading too many detective novels. Springing into action, Detective Satan begins solving mysteries left, right and center — that is until catching the perpetrator of one crime proves nearly unthinkable.