Hidden Letters delves into the lives of two modern Chinese women, Hu Xin and Simu Wu, who are connected by their fascination with Nüshu—a centuries-old secret script developed by women in Jiangyong County, Hunan province. This unique language allowed women to communicate privately in a patriarchal society that often silenced them. The documentary follows Hu Xin, a museum guide in Jiangyong, and Simu Wu, a singer from Shanghai, as they navigate personal challenges and societal expectations while striving to preserve the legacy of Nüshu. Their journeys highlight the enduring relevance of this secret language and its role in contemporary discussions about gender and cultural identity.