The Perfect Murder

"There's nothing called a perfect murder, or is it?"
3 years after Kim Seolhee went "missing" her "friends" started receiving strange messages some of them suspect that it must be the long-lost spirit of Seolhee but the others assume that there indeed was someone who knew what happened on the night of 9th September 2019.
Secrets will no longer be hidden, not until the person gets their revenge on Seolhee's "friends". Strange occurrences keep happening while they tried to find who's behind all this, soon "friends" realize that there's no escape from this. Their dark and regretful mistake will scar the rest of their lives.
You can trust no one because sometimes the answer is right in front of you.