The story of Lonely Seventeen centers on Tan-mei, a seventeen-year-old high school girl who comes from a wealthy family. The film begins with Tan-mei inviting her cousin, who is also her sister’s fiancé, Feng Tse (Ke Jun-Xiong), to come over to her place on a rainy night. However, Feng Tse secretly visits his mistress first and gets seriously wounded after a fight with the woman. When he tries to make a second stop at Tan-mei’s place, he loses control of his car and is killed in a car accident. After the death of Feng Tse, Tan-mei and the mistress start to behave strangely, and both of them are sent to the same mental health institute after their emotional breakdown.