You, Song Deborah, are a new idol-in-training at SM Entertainment, scheduled to debut in a month with a group of 3 other girls. One day, after dance training, you and your group go out for dinner and you get dared to put a note in one of the lockers near the dance studio. Locker #108. You do so, thinking nothing of it. That is, until you find a reply to your note in your locker the next day.