Whispers begin to circulate at a Catholic girls’ high school following the suicide of Unjoo one evening. Her younger sister, Jung-un, who also attends the school, becomes suspicious about the circumstances surrounding Unjoo’s death. Through determined investigation, Jung-un uncovers that on the night of the tragedy, Unjoo, along with Soy, Eugene, and Eun-young, had entered into a blood pact to end their lives together. However, while the other three survived, Unjoo did not. As the story unfolds, the underlying desires, jealousy, and deceit tied to the death pact among the four girls come to light, revealing the concealed truth behind Unjoo’s demise.