Consisting of a series of diary entries narrated by a boy over photographs made by Oshima himself during a 1964 trip to Korea, Yunbogi's Diary is a highly political tale chastising Japan's involvement in Korea.

With it, Oshima confronts his country with the mess it made during the occupation, particularly the resulting chaos and poverty that would continue beyond the Korean War. Throughout the film, he draws parallels with the situation in Japan right after World War II. The diary entries and images come across as being equally representative of Japan in post-war years. However, at regular intervals the boy's narration is halted and Oshima's own voice is heard, reading verse that emphasises the fact that this is Korea in the 60s rather than Japan in the 40s, and that children are suffering most of all. And when children suffer, the future suffers. Oshima seems to suggest that the Japanese involvement has robbed this country not only of its dignity but also of its prospects.

For much of the 60s, the director spotlighted the relationship between Japan and Korea and particularly with the way Koreans where being treated in Japan. He would raise the subject again in later films such as Sing a Song of Sex (Nihon Shunka-Ko, 1967) and Death By Hanging (Koshikei, 1968). Yunbogi's Diary was prompted by the signing of the Japanese-Korean treaty in 1965, which aimed to settle claims rising from the occupation and "normalise" the diplomatic relations between the two countries. However, the treaty only encompassed property claims, leaving much human suffering unaccounted for.

Perhaps given its openly critical attitude, the film was originally meant as a personal project of Oshima's, who planned to show it to audiences only when invited to do so. Things turned out differently and Yunbogi's Diary was eventually quite widely screened, especially outside Japan, but its non-commercial origins go a long way to explaining the film's uncommercial format, structure and running time. The technique of using sound effects and narration over still images would be employed again by Oshima for an altogether different project: the manga adaptation Ninja Bugeicho (1967).