A neglected girl falls into underground pipes while feuding with a jealous classmate. Her disappearance intertwines with an active case of three murdered women, and she helps police solve the case upon resurfacing.

Su Lin, a young girl, was neglected by her parents since she was a child and grew up in a poor and difficult environment. However, she always studied hard and looked forward to the future. Su Lin worked hard at speaking and won the role of the Little Mermaid in the English recital play, which made her classmate Ma Na jealous. During a fierce conflict, she accidentally fell into an underground pipe, and her life or death was unknown. At this time, the "case of three female corpses" in the city's underground river was also exposed to public view. In an instant, the investigation of the case was closely intertwined with Su Lin's fate. Retired policeman Gu Hao and Jiang Ting, a girl from the same school who met unexpectedly, have always been concerned about Su Lin's situation. When Su Lin finally returned to the ground, she encountered the truth about the "Three Female Corpses Case". She did her best to help the police bring the criminals to justice.