The Merrow Queen is the leader of the Celtic merfolk who reside in the waters around the Isle of Man. She was one of the first to recognize Edgar as the Blue Knight Earl, and humbly looks to him as the leader of fairy-kind.

In the nineteenth century, we find Lydia Carlton living in Scotland, making a living as a fairy doctor. She is one of those rare humans who can see and communicate with fairies. But no one believes her. However, Edgar is in need of someone with a vast knowledge of fairy lore, and Lydia is just that person. After rescuing her, he becomes her employer, but there are many secrets and emotions that seem to follow Edgar, who claims to be the Blue Knight's Earl.

Edgar Ashenbert claims to be descended from the human ruler of the fairy kingdom, and he urgently needs Lydia’s help to find and claim his birthright, the legendary sword of the Blue Knight Earl. Things will never be the same for Lydia as she is pulled into a dangerous quest against dark forces!
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(Source: Shojo Beat)