After the legendary Hong Gildong stepped down as the king of the island Yuldo, he was succeeded by his son Seong Dogun, an equally benevolent ruler. Seong Dogun, in turn, had a son who grew up and became engaged to the beautiful Lady Mihong. The era of prosperity ended when a mysterious man who called himself Jeokdae Ma'in assassinated the queen and took over the country. The king was exiled, and the crown prince went missing. Desperate, Lady Mihong sent her advisor to search for him.

Meanwhile, a young shaman named Jinsun wanders from village to village, earning his living by dealing with supernatural threats. One day he finds a gravely wounded swordsman who cannot remember who he is. Together, they help local villagers to get rid of monsters, unaware that Jeokdae Ma'in awaits them on their path.

The Romance of Forgotten Kingdom is a role-playing game set in a mythical version of old Korea and can be considered a sequel to the classic Korean novel of Hong Gildong. Gameplay-wise, it follows a traditional Japanese format, more particularly Final Fantasy games, complete with an ATB (mixture of turn-based and real-time) combat and a job system reminiscent of Final Fantasy III. Each character can switch between jobs, which imposes restrictions on weapons and equipment said character is able to use and gradually unlocks class-specific abilities called "Dao", which require magic points. During battles the player can choose to control any character at any time, since they all share the same speed bar. There is no overworld in the game, and towns are directly connected to hostile areas.