The story begins with the protagonist, who was formerly a young man named Tsutomu Yoshina, now finding himself in the body of a young girl named Tsukasa Yoshina. This transformation occurred after a week-long fever, leaving him with long silver hair, striking blue eyes (with one being red, an oddity he hides with colored contacts), and a petite, feminine physique. He struggles to accept this new reality, finding it difficult to reconcile his past self with his current appearance.
Tsukasa navigates his first day at Renchi High School, a place he previously attended as Tsutomu. He is immediately confronted with the awkwardness of his situation. His younger sister, Mei Yoshina, who is now his "older sister" in terms of their perceived age difference, greets him with a mix of familiarity and teasing. Mei, a second-year student, seems to have adapted more readily to Tsukasa's transformation, often reminding him of their new familial dynamic. Tsukasa receives love letters from both boys and girls, a testament to his new, attractive appearance, which only adds to his discomfort. He also encounters Hayate, his former best friend and now a senior, whom he must now interact with as a junior.
The narrative then shifts to a flashback, detailing the protagonist's awakening as a girl. He experiences a profound sense of disorientation, noticing the physical changes and the altered sound of his voice. Mei's shocked reaction confirms his fears. Through a series of questions, Mei confirms his identity as her brother, Tsutomu, but now in a female form. The family, particularly his mother, seems to have anticipated this change, revealing that their lineage has a history of vampires who undergo a transformation upon reaching adulthood. Tsukasa is informed that he is now a vampire, a rare occurrence in their family, and that his previous male form was merely a temporary guise. His mother, who appears to be a vampire herself, explains that this transformation is called "vampireization" and that his red eye will eventually turn blue.
The family's acceptance of this supernatural revelation is surprisingly casual, though Tsukasa grapples with the implications. His mother, a seemingly eccentric woman with a penchant for the dramatic, has already altered his legal name to Tsukasa and enrolled him in Renchi High School as a first-year student, a year below Mei. His father, though initially surprised, supports the family's decision. Tsukasa is forced to wear feminine clothing, including a frilly dress and a bra, which he finds mortifying. His mother's obsessive documentation of his transformation, including setting up a hidden camera in the bathroom, further adds to his distress.
Tsukasa's first day at school as a girl is marked by his striking appearance, drawing significant attention. He meets Ritsuka Yube, a fellow first-year student who quickly befriends him, despite Tsukasa's attempts to maintain a low profile. Ritsuka's outgoing nature and Tsukasa's own awkwardness create a humorous dynamic. He also encounters Akane, a former junior from his old club, who is now a third-year and still exhibits an overly affectionate and somewhat unsettling behavior towards him. Akane's immediate recognition of Tsukasa's cuteness and her desire to "take him home" highlight the challenges Tsukasa faces in his new identity.
During his walk home with Mei, Tsukasa's newfound vampire abilities manifest when he instinctively protects Mei from a man harassing her. He throws a pudding with surprising force, incapacitating the man. This event triggers a surge of power, causing his right eye to glow red, confirming his vampire nature. He realizes he can now access a hidden "door" within himself, which, when opened, grants him enhanced physical abilities.
The narrative concludes with Tsukasa adjusting to his new life as a female vampire, attending high school, and navigating his relationships with his family and new classmates. He grapples with the loss of his university aspirations and the complexities of his supernatural existence, all while trying to maintain a semblance of normalcy. The story sets the stage for Tsukasa's continued journey of self-discovery and adaptation in his transformed state.
