Hong Ra On (Kim Yoo Jung) is a young woman during the Joseon Dynasty who disguises herself as a man named Sam Nom to support herself by ghostwriting and giving relationship advice to lovelorn men. A love letter that Ra On writes for a client brings her in contact with Lee Yeong, who is the Crown Prince Hyo Myeong (Park Bo Gum), the first son of King Sunjo (Kim Seung Soo) and heir to the throne, as well as his trusted guard Kim Byung Yeon (Kwak Dong Yeon). Through a series of events, Ra On gets sold into the Royal Palace as a eunuch and is assigned to work for Royal Consort Soo Ki (Jun Mi Sun) and Princess Young Geun (Heo Jung Eun). Ra On becomes embroiled in a political power struggle with Queen Kim (Han Soo Yeon) and her father, Prime Minister Kim Hun (Chun Ho Jin), who somehow manage to keep King Sunjo in a perpetually weakened, hallucinogenic state. Prime Minister Kim is grooming his grandson, Kim Yoon Sung (Jinyoung [B1A4]), to take over the throne but that would put the young scholar in direct competition with his childhood friend, Lee Yeong, to become the next king. Jo Ha Yeon (Chae Soo Bin) is a minister's daughter who has her eyes set on Lee Yeong, but how will he deal with his emerging feelings for Ra On, whom he believes is a male eunuch? With Ra On and Byung Yeon by his side, can Lee Yeong fight the power-hungry ministers to claim his rightful place in history?
