Known on the outside as merely a daughter of a rich aristocratic family, 15-year-old Ririchiyo Shirakiin is far from normal – she is an Atavist, a human with demon or yōkai ancestors with the ability to channel their supernatural powers.

As Atavists are neither fully human nor supernatural, they tend to attract fully-grown, often dangerous demons, and so group together, each resident guarded by their own Secret Service (SS) bodyguard.

Unfortunately, this sudden peace is interrupted when Karuta Roromiya, Kagerō Shoukiin's SS, is attacked and heavily wounded by a fellow Atavist named Mikoto Inugami.

Inugami declares his intention to recruit members for the Night Parade of One Hundred Demons, an army of Atavists that have lost their human sanity and become senseless yōkai subject to his command.

15-year-old Ririchiyo, wrought with insecurities about her sharp tongue, leaves her home and moves into the apartment complex hoping to be alone, only to meet Miketsukami who employs himself as her SS.

Unlike other residents, Ririchiyo has no recollection of her past due to her previous incarnation's trauma over Miketsukami's death, but slowly starts to remember after encounters with the others.

The setting jumps back to 23 years ago, in the first timeline, where the group are about to bury the time capsule containing letters to their future reincarnations as a bonding activity.

As children, they had grown close, with Shimon trusting him enough to show him the Satorigahara family's Millenium Cherry Blossom Tree, a yōkai known as Sennenzakura that governs time.

An accident however, caused Inugami to become immortal, and from then on his existence began to revolve around hunting and gifting Shimon the corpses of all Atavists, for her to read their stories and memories for enjoyment eternally.

Inugami is deterred by Watanuki's presence, whom he unconsciously considered as a friend, and killed by Miketsukami, ending the Night Parade and lifting his curse, causing all senseless yōkai to return to their Atavist form.