Yuuto Amakawa, a seemingly ordinary high school student, has lived a life devoid of any supernatural encounters, unaware of his family's legacy as demon slayers. His peaceful existence takes a dramatic turn on his 18th birthday when Himari, a cat spirit and his family's guardian, appears before him. Bound by a centuries-old contract, Himari vows to protect Yuuto from malevolent spirits and demons, despite his initial reluctance to accept her presence. As a "kemonomimi" (animal-eared) girl with cat-like traits, Himari's arrival introduces Yuuto to a hidden world of spirits, demons, and supernatural conflicts.

The narrative unfolds as Yuuto navigates his new reality, balancing his ordinary life with the responsibilities tied to his lineage. Himari's protective instincts often clash with her playful and affectionate nature, creating a dynamic relationship between the two. Alongside Himari, other characters enter Yuuto's life, including Shizuku, a childhood friend and shrine maiden who harbors feelings for him, and Kuesu, a fox spirit with her own agenda. These relationships add layers of complexity to the story, as Yuuto must reconcile his growing bond with Himari while managing the expectations and emotions of those around him.

The plot delves into the ongoing struggle between humans and demons, with Yuuto at the center of the conflict due to his family's history. As the story progresses, Himari's loyalty and strength are tested, and Yuuto's understanding of his role as a demon slayer evolves. The series explores themes of duty, loyalty, and the blurred lines between humans and supernatural beings, all while maintaining a balance between action, comedy, and lighthearted moments. The interplay between the characters and their supernatural challenges drives the narrative forward, culminating in a resolution that ties together the central conflicts and relationships.