Rockfield High University is one of Seoul's most prestigious educational institutes, where people of different ethnicities enroll to make their dreams come true. But since June 1, 2015, women in their twenties have started going missing. Seoul's streets are no longer safe, and walking late at night is prohibited. Aria Heart is nineteen, half-American, and carefree. She's cute, friendly, and mostly careless of her surroundings and belongings. She misplaces her diary somewhere, and even though Aria isn't worried, her best friend plans a break-in at the university just to get it back. They never could have forseen the effects of their actions. Detective Oh Yejun is a criminal-catching prodigy. He begins his time at Rockfield High University by studying the cases of the missing girls but soon progresses to another case that leaves him completely baffled. How are the missing women related? What does Aria Heart have to do with it, and can Yejun make sense of the seeming chaos before it's too late?