A high school art teacher and closeted doujin artist named Kurokawa Kou is living a double life. She has just started a new teaching job when she encounters a talented gyaru student named Kohaku. Kohaku possesses an extraordinary gift for realistic drawing and, to Kou’s surprise and panic, is also an avid fan of her secret doujin artwork.

Flattered but terrified of having her secret hobby exposed, Kou instinctively panics and concocts an elaborate lie. She claims that the doujinshi her student loves were not drawn by her, but by her fictitious older sister, "Sachi". This desperate attempt to protect her professional reputation backfires immediately. Kohaku, thrilled to have found the creator of her favorite works, earnestly asks “Sachi” for artistic guidance. Trapped by her own lie, Kou is forced to create and play the role of her imaginary sister, becoming Kohaku’s secret art teacher.

The central storyline follows this precarious dual relationship. Kou lives as the mild-mannered teacher by day and the online mentor "Sachi" by night, guiding her talented but sometimes directionless student. As they work together, the lie becomes increasingly complicated and the bond between them deepens beyond a simple teacher-student dynamic.

Several narrative arcs drive the suspense and their relationship. The second major arc introduces the stakes of public exposure when Kohaku, wanting to include both her teacher and her online idol, suggests the three of them (Kou, Kohaku, and the fictional Sachi) attend a doujinshi convention together. Kou is thrown into a desperate struggle to maintain her disguise in a public setting where her two identities might collide. The final arc shifts the setting to the school’s cultural festival, where Kohaku has joined the art club. As they prepare for the event together, Kou must finally confront the enormous lie that has defined their relationship and decide whether to reveal her true self to the student she has grown close to. This leads to resolution concerning both Kohaku’s future aspirations and Kou’s choices as an artist and a teacher.