group of Internet chatters decides to spend the summer together at a beach, renting an apartment and trying to know each other for real (or maybe they just want to have sex). Apparently a guy named "Sandmanz" committed suicide when he was ousted from the chatroom, and the group has split opinions on the matter. Some think he was just a psycho with no life, and he had it coming. Some others, like Yoo-na (the one who got closer to him), think that someone spread lies to force him to quit. Suddenly, the group is decimated by a mysterious figure who keeps e-mailing everyone that his/her fate will not be unlike Sandmanz's. The search for the killer begins, while people die right and left. (lovehkfilm.com)

Overview from themoviedb.org