Souta Takatsuki and his childhood friend Rikka Amanome share a bond as close as siblings, yet Souta harbors feelings that transcend mere kinship. As Rikka's birthday approaches, Souta embarks on a quest to unearth the perfect present, leading him to a peculiar store where he acquires a special book without any upfront payment. Little does he know, this gift will awaken a fervor in Rikka that borders on obsession, hinting at a significant cost that may alter their relationship forever.