Culprit Among Friends

A woman, Chae Ryung is mysteriously found dead and her friends are charged as suspects. Police suspect the trio of high school friends as criminals, as they were the last known witnesses. The detective in charge of this case learns that her death is related to a case 15 years ago, and pursues the motive behind the murder. Time does not heal the pain each of the four women endure, as they turn on each other. So who exactly killed Chae Ryung?