Lee Yool (D.O.) is the Crown Prince of Joseon, a perfectionist who disregards the majority of those in the royal palace and appears to be cold and demanding, when in reality, he is lonely. He then passes the law that all women must marry before they reach the age of 28. Hong Shim (Nam Ji Hyun) is head of the first detective agency in Joseon, a strong, intelligent figure who takes on multiple jobs to support herself and her father. She is also the oldest unmarried woman in her village and begins to look for a husband to avoid trouble from law officials.
In an attempted assassination, Lee Yool develops temporary amnesia and wanders around aimlessly as a commoner with no name or identity, until he meets none other than Hong Shim, who takes him under her care. Hong Shim is then forced to marry Won Deuk to save her village from drought, due to beliefs in that era where natural disasters are a result of an unmarried woman's misery.
This drama depicts how the love story of Hong Shim, 'Joseon's oldest unmarried woman' and Lee Yool, the amnesiac Crown Prince, develops over the span of 100 days.