The story of Muoi: The Legend of a Portrait is about a young woman named Yun-hee. She is a South Korean writer who is currently pressured by her editor on producing her next book. Yun-hee hasn't been able to publish a single book single three years after her last book. During this time, Yun-hee becomes desperate to find inspiration in looking for a new story to write about, until one day, her old friend Seo-yeon gives her a call.
Seo-Yeon tells Yun-hee about a local Vietnamese story based on a girl named "Muoi" and her portrait that is considered to be haunted.
After hearing this story from her friend, Yun-hee becomes very interested in finding more information on this girl and her portrait that Seo-yeon sends to her. At this point, she becomes incredibly impressed with what she finds with this information, which makes her become even more inspired to create a story based on Muoi.
Surprisingly, Yun-hee discovers that the previous novel she wrote that was called "Secrets & Lies", that was based around her friend, Seo-Yeon and her personal life. In this book, Seo-Yeon was portrayed in a negative light, but her friend hasn't yet read this book since she was living in Vietnam for several years. Yun-hee flies out to Vietnam to find out more about Muoi.