Set during the Joseon Dynasty period, Haechi follows four people from different walks of life come together to make Prince Lee Geum the King and reform the Saheonbu. The four people are Lee Geum (Jung Il-Woo), Yeo-Ji (Go Ara), Park Moon-Soo (Kwon Yool) and Dal-Moon (Park Hoon).
Lee Geum is a prince, but, even though he is smart, he is not recognized as a prince due to his mother being born in the lowest class.
Yeo-Ji is damo at the Saheonbu. She investigates cases enthusiastically. She has a talent for the martial arts and foreign languages. Yeo-Ji is the best investigator at the Saheonbu including the male investigators.
Park Moon-Soo has been preparing for his exam to become a public officer. He is not very smart and has difficulty with the exam, but he is honest and tender to other people.
Dal-Moon is a famous clown. He is good at martial arts.