Sabaki no Chi

In the castle, a poor boy, Biskyu Cris, tries to steal a medicinal herb which can extend the life span of the Castral people who only live for 25 years. Biskyu is saved by a tall beautiful, person completely covered by a long coat.
After a lengthy period of sword training, Biskyu returns to the castle to become a knight and guard the person who originally saved him. Biskyu believed the person was a woman and wanted to sacrifice his life for her. However, his savior was a *king* of Castral, Erudora, who's 700 years old. (Erudora contracted with the Gyokuto people who have magical powers to cure any diseases. In return for his blood, Erudora received eternal life and awful blue blood which grows the medicinal herb.)
Biskyu has the opportunity to become a knight Erudora's daughter, but instead falls in love with Erudora again. Biskyu, learning of the conspiracy by the Minister of Justice to try and kill Erudora, escapes with Erudora to release him from his accursed fate...