The film is based on a same-titled book by novelist, Lisa See, in 2005. Set in 19th century China, the film depicts a lifetime friendship between two women who developed their own secret code as a way to communicate in a reclusive life under rigid cultural norms.

The words nu shu literally means "Woman's Writing" in Chinese. As the name implies, Nushu is a writing system created and used exclusively by women in a remote part of China. Traditional Chinese culture is male-centered and forbids girls from any kind of formal education, so Nushu was developed in secrecy over hundred of years in the Jiangyong county of Hunan province.

Some Nushu characters are taken from Chinese, while others appear to be invented, but all are rendered in a style much more cursive than written Chinese. In addition, the characters are "thinner" than Chinese characters, which tends to be square-shaped. Also, like Chinese, Nushu is written from top to bottom in columns, and the columns are written from right to left.