Ayana Ushio, who married a divorced man much older than her, began living with his son Tatsuya, born from his previous marriage. Since Ayana and Tatsuya were close in age, they quickly became close, bonding like siblings. However, after Ayana safely gave birth a few months into her pregnancy, welcoming the newborn into the family, Tatsuya began feeling left out and isolated. This growing sense of exclusion gradually changed his behavior, making him act strangely.