Natsuki, a sensitive girl growing up in a world she struggles to comprehend, finds solace and meaning in a secret identity: she is not an Earthling, but a magician from Planet Popinpobopia, sent on a special mission to save this world. Her cherished stuffed hedgehog, Piyyut, is her guide and confidant, the only one who truly understands her extraordinary purpose. This fantastical inner life becomes her refuge from a family that often leaves her feeling like an outsider, particularly her mother, who seems to favor her older sister.
Summers spent in the wild mountains of Nagano offer a reprieve, where she reunites with her cousin, Yuu. He is the only other soul who seems to perceive the world with the same alien eyes, and soon, Natsuki shares her deepest secret with him. Yuu, in turn, confesses his own belief that he is an extraterrestrial, possibly from the very same planet as Piyyut. Their shared delusion solidifies into a sacred bond, a pact to survive together against the incomprehensible demands of the "Baby Factory" that is human society, where everyone seems compelled to conform, marry, and procreate.
But childhood innocence is a fragile thing, and Natsuki's world is brutally fractured by the harsh realities of the human realm. She endures abuse from her family and is preyed upon by her cram school teacher. These traumatic experiences further entrench her belief in her alien identity, leading her to develop dissociative coping mechanisms. The magic she once imagined becomes a necessary out-of-body power, allowing her to escape the pain by observing herself from a distance, as if her true self exists elsewhere. An incident during one of their summer visits, a moment of desperate intimacy between Natsuki and Yuu, is discovered and violently punished, shattering their immediate connection and reinforcing their alienation.
Years later, Natsuki, now an adult, navigates the "Baby Factory" with a carefully constructed façade of normalcy. She enters into a marriage of convenience with Tomoya, a man who shares her asexual nature and her profound disdain for societal expectations. Their union is a performance, a shield against the relentless scrutiny of family and friends who pressure them to conform, to have children, to be "normal." Internally, Natsuki still clings to her Popinpobopian identity, her childhood fantasies evolving into a radical rejection of the human world she feels utterly detached from.
The pressure mounts, and the dark shadows of her past begin to close in. Natsuki's carefully maintained existence unravels, pushing her to the brink. In a desperate bid for survival and authentic selfhood, she flees the suffocating suburbs and returns to the wild mountains of her childhood, seeking refuge and reunion. There, she hopes to find Yuu, to rekindle their shared understanding and the promise they made long ago.
In the isolated mountain retreat, Natsuki, Yuu, and Tomoya converge, their shared sense of otherness forming an unholy alliance against the world. They shed the pretense of humanity, embracing their "alien" identities and the radical freedoms they believe come with it. The world they perceive is one where humans are merely cogs in a reproductive machine, and they, the Popinpobopians, must find their own grotesque means of sustenance and survival, far outside the bounds of Earthling morality. Their journey culminates in acts of shocking rebellion, a bizarre and visceral defiance of everything considered human, all in the name of remaining true to their perceived extraterrestrial selves.
