At a girls' high school run by a certain church, four pure and beautiful girls, Yan Zhu (played by Zhang Jingya), Yin Suyi (played by Sun Yinsu), Li Youzhen (played by Wu Yanrui), and Jin Enying (played by Song Minjing), are friends in different classes of the same grade, with Yan Zhu and Suyi being especially close best friends. One night, the four girls vow to 'not live together, but hope to die together,' planning to commit suicide. However, in the end, only Yan Zhu jumps from the rooftop of their school. After the incident, Suyi and the others agree to keep the secret, both to avoid trouble and to escape the ridicule and contempt of their classmates in case of a failed suicide. But isn't there a saying that walls have ears? Soon, the news of their suicide pact with Yan Zhu spreads among their classmates. On the other hand, a dark shadow seems to be lurking around them, as if fate is watching over them. Can a vow be made lightly? And how can it be easily broken?