In a kingdom gripped by turmoil, privileged seventeen-year-old Iseul defies danger to rescue her sister Suyeon from the absolute power of tyrannical King Yeonsan, while Prince Daehyun, desperate to overthrow his despotic half-brother, forms an uneasy alliance with Iseul, leading them to join forces in a perilous gamble against the oppressive regime.

When the children find a letter written in riddles in Aunt Jane's attic, they believe it may contain clues to a hidden treasure on the island, but they soon discover they are not the only ones treasure hunting.