Rinka has secretly been sleeping with her son's adult friend Kitayama for five years. It began after her husband's affair was exposed and Kitayama comforted her with a tender kiss. She waited until his graduation, when he was an adult, before answering his feelings and starting their sexual relationship. They continued meeting behind her husband and son's backs, but after Kitayama gets a girlfriend, their long-running creampie affair begins to change.