In the 1990s, life in a city along the river seemed peaceful, but Su Lin, a young girl, always wanted to seize the opportunity to change her destiny.She carefully prepared for school activities and practiced her spoken English repeatedly to win the role of “小美The Mermaid”.For Su Lin, this role is not only an opportunity to appear on stage, but also a little hope for her to prove herself and temporarily escape from her repressed life.

Su Lin's efforts did not result in an easy campus life.There was a gap between her and her classmates that was difficult to bridge, and conflicts surrounding her role and daily interactions were also accumulating in the dark.Some people regard her persistence as a way to get the limelight, while others use jokes and rejection to test her bottom line.Su Lin did not back down immediately, but she also did not realize that a malicious attack on her was crossing the boundary of ordinary classmates' conflict.

The conflict that day suddenly got out of control.Su Lin was violently pushed down in the chaos and fell into the intricate drainage pipes underground in the city.People on the ground quickly lost sight of her, and an ordinary day turned into an unexplained disappearance.The dark and damp underground space separated Su Lin from the familiar streets, and for the first time she truly faced the fear of having nowhere to turn to.

Su Lin's disappearance alarmed her family, school and surrounding residents.Some people are eager to find her, some are avoiding what happened that day, and some are trying to turn the conflict into an accident.As the search begins, the neglected details of Su Lin's life gradually emerge: the silence at home, the cold eyes on campus, and the distress signals that were not spoken in time all make the disappearance case more complicated.