In 15th century China two women are born under the same sign, the Metal Snake. According to Confucius, 'an educated woman is a worthless woman', but Tan Yunxian -- born into an elite family, yet haunted by death, separation and loneliness -- is being raised by her grandparents to be of use. She begins her training in medicine with her grandmother and, as she navigates the male world of medicine, requiring tact and diplomacy, she struggles against the confining world of her class. From a young age, Yunxian learns about women's illnesses, many of which relate to childbearing, alongside a young midwife-in-training, Meiling. The two girls find fast friendship and a mutual purpose and they vow to be forever friends, sharing in each other's joys and struggles. How might a woman like Yunxian break free of tradition, go on to treat women and girls from every level of society, and lead a life of such importance that many of her remedies are still used five centuries later? How might the power of friendship support or complicate these efforts?