The Demon Slayer Corps plunge into Infinity Castle to defeat Muzan. However, the remaining Hashira and the Demon Slayers who survived Tanjiro's Final Selection are pitted against the last of the Twelve Kizuki first.

The modern-day bizarre Romanesque story follows Sumireko Ogawa, an aspiring novelist who loves mysteries and works at a bookstore with a boy named Ren Adashino. They witness strange occurrences and attempt to solve mysteries.