Yoji confides in his wife's infidelity to his mother-in-law, Mari. Drawn to her large breasts, he initiates an affair, but Mari firmly resists, refusing to surrender to Yoji's advances due to their previously sexless marriage. However, her heart gradually warms toward him, and their relationship deepens. The story unfolds with complex emotions entwining the forbidden bond of an affair with a stepmother and rising desire.