Karen is an ordinary high school student in Tokyo, but she sometimes recalls pieces of her childhood memories that she did not know before. During the process of getting her passport for a school trip, Karen views her family register. She learns that she had an older sister Yukari, who died at the age of 4. Karen also learns that her parents are not her biological parents and that she was adopted. Karen now wants to learn about herself. She lies to her parents and tells them that she is leaving on the school trip. Instead, she travels to Fukuoka where she was born. Her classmate Satoi Hogari accompanies her.
The criminal library, dubbed the “Red Museum”, is a police facility that stores the investigation materials and evidence of major crimes where the statute of limitations has run out. Its director Saeko Hiiro is impassive and not good at communicating with people. One day, Satoshi Terada of the Tokyo Metropolitan Police Department’s First Investigative Division is assigned to be Saeko’s subordinate. Although he is bewildered by Saeko’s dispassionate demeanor, Terada starts to organize the data which is the primary responsibility of the museum. He encounters a traffic accident on the way to collect evidence. A truck has hit someone. When Terada rushes over, the victim utters the mysterious words “25 years ago... murder swap” and breathes his last. Once Saeko hears this, she declares a reinvestigation.
