Kim Hye In is an unconventional student at Cheongdam International High School due to her lower-income background – and because she's now witnessed two deaths of students related to the school. She's also the newest member of Diamond 6, the school's most elusive clique, alongside Baek Je Na, the leader of the group and the prime suspect in one of the deaths Hye In saw. However, Hye In's membership in the Diamond 6 and the introduction of new students shake up Cheongdam High's social hierarchy, threatening Je Na's formerly stable position at the top