When Min-Ah discovers secret shared diary written by two students at the all-girls' school, she finds it a compelling read. She becomes obsessed with the page-turning accounts of a secret romance between the two girls, but when one of the students is found dead, a new terror seeps through the school. Was it suicide or something more sinister? And what is the true meaning of the words written in the diary, "memento mori" (remember the dead)?