Sixteen-year-old Haruka lost her mother young, receiving a hand mirror as a final keepsake. Over time, she forgot and lost the mirror, deepening her emotional detachment and straining her relationship with her workaholic father. Feeling neglected, she visits a shrine praying for the mirror's return, recalling a folk tale about foxes collecting neglected items. There, she encounters the fox-like Teo, who steals her keychain. Pursuing Teo, Haruka follows him through a portal into Oblivion Island, a realm inhabited by creatures gathering forgotten human objects.

Initially single-minded and stubbornly focused on retrieving her mother's mirror, Haruka's journey forces her to confront neglected aspects of her past. She encounters her forgotten childhood stuffed animal, Cotton, who voices resentment for her abandonment. Her interactions with Teo and Cotton, alongside challenges from the island's ruler, the Baron, drive her emotional growth. She experiences betrayal when Teo aids the Baron for personal gain, but witnesses his eventual redemption. Risking herself to save Cotton during a clash with the Baron marks her shift from self-interest to protective compassion.

The magic mirror acts as a conduit for her repressed memories. Upon retrieving and activating it, she is overwhelmed by visions of her family life, especially poignant memories of her mother. This allows her to process grief and guilt over forgetting the mirror and associated aspects of her mother. Her adventure concludes with renewed perspective. Returning to the human world with Cotton, she actively reconciles with her father, signaling the resolution of their estrangement by calling him for dinner.