Based on Misuzu Nakahara's allegedly "autobiographical" novel in which the writer claims that she participated in the unsolved crime widely known in Japan as "Sanoku-en jiken (300 Million Yen Affair)" when she was a high school student.
Although the story depicted in the film is fictitious, the crime actually took place on December 10, 1968. The amount stolen in a single crime is still the highest record in Japan (equivalent to today's $2M), and despite more than 100 evidences left behind, with 170,000 policemen against 110,000 suspects spending almost a billion yen in the investigation, nobody was arrested and the stolen money was never found. The investigation was officially closed for prescription in 1975.