Gym teacher Ritsuko Morita gets into an accident while horse riding that results in her stallion's death. A few days later, Ritsuko discovers that she has grown horse genitalia! Baffled by her situation at first, she soon finds herself unable to stop masturbating, as the pleasure is beyond anything she has ever experienced.
Her situation soon grows even worse when she begins admiring her female students' bodies and imagining herself in various erotic situations with them. One evening, no longer able to fight her urges, Ritsuko forces herself on a female student that is on her way home.