Jin-soul, who is just beginning to gain a reputation as a writer with her novel, Human in a Natural State, meets a woman, named Nan-ryeong, at a reading session held at a local bookstore in Gangneung. Nan-ryeong has also written a novel, titled 'The Women Who Brush Their Teeth Outside', influenced by Jin-soul. Jin-soul, who feels a strange sense of connection to Nan-ryeong, reads her novel on the train returning to Seoul. A story of two people who somehow overlap. But, while Jin-soul has been cleverly hiding behind a pseudonym, Ju-ok, Nan-ryeong is open about who she really is. Eventually, the insidious relationship between the two is exposed, and cracks begin to form in Jin-soul′s seemingly happy life.