Kiku-chan, a cryptic figure cloaked in traditional Japanese attire, materializes within the World Border—an ethereal alternate dimension—to guide Mio, a girl stranded there after a traffic accident severs her from her physical body. Having entered the realm through her own out-of-body experience, Kiku-chan reveals that escape demands both a train ticket and an unwavering, deeply felt "desire" to return to the human world. Alongside Mio, a plush companion named Gimon, and Arata, Mio’s childhood friend, she strategizes to bypass Mogari, the guardian obstructing their path.

A pivotal truth emerges: Kiku-chan is the sentient manifestation of Mio’s former pet fish, Kii, whose bond to Mio in the human world fuels her determination to secure the girl’s safe return. Though Mio initially fails to recognize her, their shared history lingers beneath the surface, gradually resurfacing as they traverse the Border’s challenges. Kiku-chan’s guidance proves instrumental in Mio’s journey, not only in recovering splintered memories but also in confronting buried emotions tied to Arata.

Interactions with entities like Mrs. Mori, a goddess overseeing the Border, and a meticulous lost-and-found officer underscore Kiku-chan’s role as a mediator between the tangible and the metaphysical. Her presence weaves together pragmatic problem-solving and emotional revelation, anchoring Mio’s resolve to reunite body and spirit.