Taking advantage of his father's absence, Koichi makes a move on his stepmother, Sumire. Recently stressed from work and sexually frustrated, Sumire realizes that Koichi's touch excites her most sensitive spots. Obsessed with the big-titted, alluring Sumire, Koichi escalates their relationship into forbidden territory. Fearing discovery by his father, the two continue their affair in secret. Drawn deeper into this taboo relationship, they gradually form an increasingly intense bond.