The Prince never found Cinderella, the girl who lost a glass slipper, but he never did forget her either. What he failed to be aware of was the fact that she was searching for him too. He hid his sadness with his face and she hid her face with a mask. 

She's slowly tearing the thin strand of sanity apart. 
She's an assassin. She's also an agent of the Red Force, an association dealt with scattering fear among the inhabitants of EXO Planet and gaining power, eventually taking over the thirteen kingdoms. Her true identity is unknown, and she hides her face with a mask. Because of that, she is known to others as "No Face". 

Now's her turn to be the guardian of her guardian angel. 
Jung Juniel's oftener regarded as a commoner, but in truth, she's spent her whole life hiding a dark little secret of her family's. She's spent her whole life telling lies, that in fact, she can't tell from truth or lie anymore. She's been living under the shadow of her long lost sister, Jessica, who is long gone since that day. She knows that Jessica is out there, waiting for her and she's determined to find her no matter what. 

With true love comes many obstacles.
The princes are in no better situations themselves. Each is faced with the same dilemma as one another. As heirs to the throne, they are required to find a suitable partner in life before being crowned ruler of their kingdom. 

A faceless assassin with an unknown past, a girl determined to find a long lost sister, and twelve princes who grew up being taught to despise each other and to avoid interaction with the lesser social classes of people.

A world full of magic, a world where a snap of a finger might as well be the difference between life or death, peace or war, where you either outlast others and withstand pain or just simply cease to exist.
The Fates made several knots with numerous different strings that couldn't they couldn't undo. They were destined to meet. But whatever happens to the rest of the string is up to whatever choices they make.