In the summer of 2002, when Korea is abuzz over hosting the FIFA World Cup, eighteen-year-old Kim Hae-on is killed in what becomes known as the High School Beauty Murder. Two suspects quickly emerge: rich kid Shin Jeongjun, whose car Hae-on was last seen in, and delivery boy Han Manu, who witnessed her there just a few hours before her death. But when Jeongjun's alibi checks out, and no evidence can be pinned on Manu, the case goes cold. Seventeen years pass without any resolution for those close to Hae-on, and the grief and uncertainty take a cruel toll on her younger sister, Da-on, in particular. Unable to move on with her life, Da-on tries in her own twisted way to recover some of what she's lost, ultimately setting out to find the truth of what happened.