The story revolves around a young girl, during the early 20th century, trying to adjust to her parents' divorce and joint custody--she spends half of each year with each parent, traveling between their cities of residence. Her parents do not agree on any subject, including her mode of dress, her indulgences, her behavior, or even on what to call her; her father calls her Mary, her mother calls her Marie. In her diary, Mary Marie decribes her feelings on everything affecting her life, including her preference for her mother's frivolous lifestyle and rejection of her staid father's rigid rules and dress code. Unfortunately, from Mary Marie's point of view, her mother begins a discipline regime, explaining that it is for her own good.