Endowed with formidable supernatural power from birth, Un Jonghyeon defied his rank as the seventh prince to become the clear heir to the throne. His solitary existence in the imperial palace is interrupted by the sudden arrival of a new servant, Yuri. "Swear to serve me until your last breath." Jonghyeon finds himself drawn to the seemingly innocent Yuri, whose presence is strangely captivating, and he slowly begins to let down his guard. But as three years pass, the day of Jonghyeon's critical coming-of-age ceremony draws near...