Secret Door (Korean: 비밀의 문; RR: Bimir-ui mun) is a 2014 South Korean television series starring Han Suk-kyu, Lee Je-hoon, Kim Yoo-jung, Park Eun-bin, Kim Min-jong, and Choi Won-young. It aired on SBS on Mondays and Tuesdays at 22:00 from 22 September to 9 December 2014 for 24 episodes.^[1][2][3][4]

The period drama explores the conflicted and ultimately tragic relationship between King Yeongjo and his son, Yi Sun (Crown Prince Sado). Shrewd Yeongjo wants to strengthen royal power, but passionate and idealistic son dreams of equality and a status-free society.^[5]

Secret Door is a revisionist take on the last eight years of Crown Prince Sado's life.^[2]

According to historical documents, primarily those written by his wife in the Memoirs of Lady Hyegyeong, Sado was insane and violent, and was a serial rapist and killer. He was deemed unfit to rule, but court rules forbade Yeongjo from directly killing royalty. So Yeongjo issued a royal decree that ordered Sado to climb into and be sealed within a large wooden rice chest on a hot July day in 1762. After eight days, Sado died of suffocation and starvation at age 27. In the 19th century, conspiracy theories arose that Sado had not been mentally ill, but had pushed for revolutionary changes and thus his political opponents had framed him for crimes he did not commit. The circumstances surrounding his death continue to be an issue of debate among Korean historians.^[1]

Upon his ascension to the throne, Sado's son King Jeongjo memorialized and honored his father's tomb, and Sado and Hyegyeong were posthumously elevated in status and given the titles Emperor Yangjo and Empress Heonyeong.^{[citation needed]}