Natsumi Rokudo, a ten-year-old Japanese schoolgirl, receives an ancient New Guinea statue from her archaeologist father. This statue unknowingly contains the sealed spirit Ongo. Accidentally breaking the seal leads Natsumi to encounter the earth spirit Ahem in a dream. Ahem teaches her a ritual dance enabling her transformation into Mii, a powerful fertility goddess.

Manifesting as an adult woman, Mii possesses dark skin, short pale blonde hair, gray eyes, and a notably voluptuous figure. Her physical form features a skull necklace, a leaf-adorned orange bikini top, a yellow-red skirt, and green ballet shoes, symbolically representing Earth's life-giving energy. Her large breasts explicitly contain this power. The transformation process involves specific bouncy and suggestive dance movements accompanied by intensifying vocalizations. Upon completing the dance, Natsumi fully assumes Mii's appearance and abilities.

Mii's powers encompass flight, control over plant life, and gigantification. During gigantification, her entire body enlarges, with her breasts expanding proportionally earlier than other parts. This ability proves crucial in confrontations, such as the final battle against Ongo's awakened demon form. In that battle, Mii defeats Ongo by calming his rage, enveloping him with her exposed breasts after her clothing sustains damage.

Natsumi's journey involves reconciling her identity with Mii's form. Initial transformations cause embarrassment due to attention drawn to Mii's physique and the dance's suggestive nature. Her classmate Takuma exemplifies this tension, idolizing Mii while showing no romantic interest in Natsumi. The transformations parallel Natsumi's own coming-of-age experiences, including her first menstrual period. Mii occasionally exhibits an independent presence, communicating with Natsumi during transformations or gigantification.

Beyond the core Ongo conflict, Natsumi engages in varied adventures across expanded media. These include thwarting a robbery, managing chaos caused by transformed cafeteria food, navigating alien encounters, confronting supernatural entities at a hot spring, and preventing a friend from exposing Mii's secret. She also confronts Rongo, a water spirit possessing her friend Nami, who challenges Natsumi due to jealousy over Ongo. After Rongo's defeat, she cohabitates with Natsumi, adding ongoing interpersonal dynamics.

Natsumi's development reflects themes of burgeoning responsibility and maturation through her dual existence as an ordinary schoolgirl and a goddess embodying Earth's vitality. Her experiences span comedic, perilous, and fantastical scenarios as she balances human relationships with her divine role.