Several girls who used to live in the same dormitory are now over 30 and faced with various problems in their own lives, careers and marriages. A sudden gathering brings them together again but none of them know that things are not as simple as it seems. A few days earlier, Lin Ji (Fan Shiqi) who is hosting the party received a video from 15 years ago. It was a recording that belonged to Jiang Yuanyuan, a girl who was once their roommate. During that summer, she committed suicide and in the video, she mentions feeling devastated because a best friend had betrayed her. The incident has cast an unforgettable shadow in the lives of the former schoolmates. Lin Ji repeatedly brings up the topic of Jiang Yuanyuan's death which the girls kept evading. As one of them attempts to leave, they realize that the door has been locked from outside. What happened that nightmarish summer 15 years ago and why did Jiang Yuanyuan die?