Childhood friends Airi Nagisa and Higashi have always had a rocky relationship, constantly bickering due to their sharp tongues. When Higashi enters high school, he starts his first romantic relationship with a female classmate and begins feeling confident about himself. Unbeknownst to him, Airi secretly learns from his girlfriend that Higashi suffers from premature ejaculation and is struggling in bed, stirring up complicated emotions within her. To help his girlfriend, Airi offers herself to Higashi as a practice partner under the guise of "training" for oral sex. Is she doing this out of mockery, jealousy, or some deeper, hidden feelings? The truth slowly begins to unravel...