Mona and Dee-Dee apparently have only one thing in common: their father! After growing up far from each other, they learn the truth when they come to live in the same building in San Francisco. They then discover a new, unique and strong fraternal bond; even if their cohabitation is very eventful... Raised by her single mother, Mona is an independent and fundamentally free young woman. In charge of production in a record company, she has a strong will and a strong character! In contrast, her younger sister Dee Dee Dee is a model girl who has had the privilege of growing up in a two-parent family; but she has difficulty with her mother's omnipresent authority. Recently graduated in law, she is looking for a job that will allow her to obtain her independence... Between the two sisters is Spencer, Mona's friend and colleague, who tries to keep the peace by acting as the voice of reason for both of them; even if he is secretly in love with Mona...