Narifumi's mundane school life is upended when Shizuku crosses a forbidden portal to seek ultimate power, dragging Narifumi and Kasumi into a perilous parallel dimension. As they battle strange creatures in this new world, Narifumi is approached by a mysterious entity who offers him the strength to rule the world in exchange for his essence, thrusting him into a confusing and erotic struggle for survival.
Narifumi and his companions discover the truth about the Black Gate, a portal to a ruined realm destroyed by a catastrophic force known as "The Power to Rule the World." While the Kadomori family and their fellow guardians strive to keep the seal intact, Shizuku betrays her duty to seize this forbidden power for herself. Following a cruel attack that leaves Mizuki traumatized and Sora captured, Nanami transfers her ancestral knowledge to Hosokawa, entrusting him to lead the remaining guardians in the fight to come.