Osan is a prostitute within the famous Yoshiwara pleasure quarter, sold into the profession as a child. Sankatsu, her sister, is a respectable married woman and mother. The two sisters are separated by their situation in life, confined within their societal roles, and bullied by vengeful and domineering women; in Osan's case it is the brothel keeper's wife, and for Sankatsu it is her overbearing mother-in-law. Desperate to be reunited, they hatch a plan to end their misery and escape their respective prisons. Told through the correspondence exchanged between Osan and Sankatsu, and the letters exchanged by those close to the two sisters, My Dearest Osan is a tale of love, duty, betrayal and revenge in 19th century Japan.