Twenty-seven years into the future, the weaponization of demon tentacles throws the entire world into the chaos of war. Meanwhile, the demons who developed romantic feelings for the Idaten each meet an unexpected end. As the world heads down a path of reconciliation and coexistence under the leadership of Miku, only Gill harbors doubts. When the Demon Lord's identity comes to light and the ancient seal is about to be broken, the final battle of the mightiest Idaten begins!