Nobody Knows tells the story of four siblings and their young mother (played by J-Pop star You) who move into a small apartment in Tokyo. There is no father around, and in fact, all the children are the offspring of different men, as a result of their mother's wild lifestyle and immature personality. Yet, in the beginning, the family life is portrayed in a lovingly chaotic way. Society might not give theseal of approval, but the members of this non-nuclear family seem to enjoy an adventurous life together.
Things change for the children—12-year-old Akira, his serious-minded younger sister Kyoko, and their two younger siblings: avidly curious Shigeru and the adorable and innocent Yuki —when, shortly after they move in to their new home, their mother leaves to begin a new life with a man who lives in Osaka. Left with only a small amount of money and a note asking Akira to look after his younger siblings, the eldest brother's rank in the family shifts overnight and he becomes the surrogate head of the family.