Since his divorce when Takumi was 5 years old, his father, who had been raising him alone, has remarried. He has begun living with his stepmother, Miki... Miki is a caring Miki who tries to close the gap between Takumi and him by getting involved in his affairs, and they even end up taking a mixed bath together. Takumi, unable to resist her, accepts... However, the stimulation of having his body washed is too much for the virgin Takumi, and he explodes without being able to resist. Miki gently embraces him as he trembles with shame. This is the start of secret practice in the bathroom with his stepmother...