As a child, Seo-woo rejected Won-hyeok, the boy who once led her into the darkness of a bunker. Years later, now a high school student living a life of bitter isolation, Seo-woo finds herself driven to the edge. In the middle of an attempt to end it all, she stumbles upon another abandoned bunker and steps inside to hide. But her sanctuary becomes a prison when she is trapped inside by none other than Won-hyeok—who returns drenched in fresh blood.