In a world where humans are locked in a violent conflict with monstrous creatures known as Mimushi, a young sorcerer named Mashiro ventures into dangerous, insect-infested mountains on an extermination mission. There, he stumbles upon a strange sight: a mysterious girl named Yuren living quietly in a cabin with her mother on this hazardous hill, seemingly unbothered by the swarming bugs around them. Mashiro is initially shocked by their presence, but he soon realizes there is far more to Yuren than meets the eye.

The truth is that Yuren and her mother are not ordinary humans. Her mother, Toya, is revealed to be a powerful Disaster-Level Mimushi, and Yuren herself is a hybrid, half-human and half-insect. Despite her dangerous nature, Toya has resisted the instinct to kill humans, only fighting back when sorcerers threaten her family. After a tense confrontation, Mashiro and Toya reach an understanding. Rather than fighting to the death, Mashiro agrees to let the sick and dying Toya pass away in peace. In return, he takes on the responsibility of caring for Yuren, protecting her and helping her try to live a normal human life despite her monstrous heritage.

The story follows the unlikely pair as they navigate a society that would see Yuren as nothing but an enemy. Mashiro, whose name is also given as Miyaji, works to acclimate the naive and sheltered Yuren to human society while helping her control the bloodlust that comes with her Mimushi nature. Their bond forms the core of the narrative, a found family dynamic that is repeatedly tested by external threats. Other humans who learn Yuren's true identity are divided on how to handle her, with some calling for her death and others proposing dangerous experiments to understand her abilities. The plot escalates rapidly from intimate character moments to large-scale conflicts, including infiltration fights and even a civil war, as the young girl caught between two worlds becomes a catalyst for a much larger upheaval.