A television series talk about Jang Yeong-sil (Song Il-gook), a popular scientist of Joseon in the 15th century. Born as a slave but with the remarkable talent and the profound passion for astronomy, he was known by King Sejong (Kim Sang-kyung) and was recommended to the court. Jang Yeong-sil had a lot of contributions for the science-technology of Joseon, he has left many great inventions such as the water clock, the sundial, the astronomical instruments, the rain gauge and also many research on weaponry.