Ero gibo to hatsujô musuko: Midarana kazoku (2004)

From the outside, Yuriko appears to be living a happy life. However, her husband is away on a business trip, and what's more, he seems to be having an affair with her secretary. And although he's her step-son, he's a completely grown man, and she lives alone with him. Sensing a forbidden desire reflected in her son's gaze towards her, she feels a sense of immorality and guilt, but her ripe body aches mysteriously. Then, Yuriko finds out that a widowed mother and son who live nearby are having a physical relationship. And that night, driven by an uncontrollable ache, she visits her son's bedroom...