Takeshi Ninno appears to have a normal relationship with his stepmother and stepsister, Shino and Shinobu Oshino. However, behind closed doors, he and his stepmother have an illicit relationship where she forces him to engage in sexual activities with her. Troubled by the acts he is compelled to perform, Takeshi wishes to end the affair—but he also does not want Shinobu to hate him for the intimate pastime he and Shino partake in. The situation is even further complicated when his other stepsister, Fumino Oshino, unexpectedly comes back into the picture. [Written by MAL Rewrite]

Chibo was released on DVD in English as Mother Knows Breast by Kitty Media on July 25, 2006.