Chihiro and her son Tetsuya have lived together as a family of two since the early death of her husband. Left to raise her child alone, Chihiro has worked tirelessly to protect him. Tetsuya, vowing to protect his mother in place of his late father, gradually finds his feelings evolving into something forbidden. One day, they decide to go on a trip together. The thought of spending uninterrupted time with his beloved mother fills Tetsuya with anticipation. However, during the trip, Chihiro introduces him to her new boyfriend, Tohru, leaving Tetsuya shocked and deeply unsettled. Unable to accept the polite yet threatening presence of Tohru, Tetsuya grows increasingly hostile. Then one day, he catches Chihiro meeting Tohru in secret. This discovery becomes the catalyst that shakes Tetsuya's heart to its core.