Three students. One school. One killer. When three high schoolers are found murdered with brutal marks carved into their backs, rumors of a serial killer-called "X"-start to spread. Lee Hana, a sharp and strong-willed detective, is forced to go undercover as a literature teacher at Baekho Academy. She's here to investigate. To find the truth. To blend in. But between nosy students, fake smiles, and secrets buried under polished hallways-she finds something worse than a killer... A charming, carefree science teacher named Seo Joon, who seems too relaxed in the middle of a murder investigation. As Hana dives deeper into the case, nothing is what it seems. The school is hiding something. The students are scared. And worst of all... The killer may be closer than she ever imagined. A mystery with romance, betrayal, hidden identities, and the one truth no one dares to guess. Who is Killer X? You'll never guess.