Ten years ago, Natsumi divorced her husband and was separated from her son, Taichi. In her loneliness, her feelings for Taichi gradually transformed from mere maternal love into an abnormal, obsessive desire. One day, her ex-husband died in an accident, leaving Taichi with no family, so he returned to live with Natsumi. Dreaming of rebuilding their family life, Natsumi invited Taichi on a hot spring trip. During their journey, overwhelmed by affection, Natsumi clung to Taichi's body and instinctively pressed herself against him. That single moment became a symbol of the deep-seated loneliness and obsession buried within her heart.