Cheon Sooah is a Gemless, the lowest social rank at Boseok Academy. After being adopted by her estranged grandfather, she suddenly finds herself not only as the envied and bullied highest ranking female on campus, but also as the fiancée of two possible suitors in an arranged marriage: Kim Jongin and Oh Sehun, the Academy's most high ranking, sole Black Diamonds. While she can't escape the promises that had been made for her long before her birth, Sooah realizes that the decision of whom to marry is ultimately hers to make. 

There are four social classes at Boseok Academy

White Diamonds, of old money and family names packed with wealth.

Jades, of new money and families that made themselves a name. 

Gemless, of those with a name, but no fortune. 

Above them all,

Black Diamonds, a rank consisting of only two students, both heirs to two of the country's top three financial pillar clans.

Upon entering Boseok Academy, every student is given an ear-stud according to their social class, obliged to abide to the strict social hierarchy and follow the Academy's law:

"Black Diamonds the sky covering all. White Diamonds the stars illuminating the night.

Jades the earth to walk upon. Gemless to look up and admire them all."

After her parents' death, Gemless Choen Sooah is taken in by her runaway mother's estranged family.

Upon entering the household she learns that not only she is now a member of the country's richest clan, but also promised to one of two possible suitors of a marriage that had been arranged for her by her grandfather and his two most entrusted friends long before she had been born. Faced with an unavoidable fate, which her late mother had tried to protect her from, Sooah reenters Boseok Academy with a new rank and broken heart. 

As the only one wearing a White Diamond with a black heart, Sooah is confronted not only with campus-wide outrage, but also the wrath of her only aunt's daughter, her cousin Eunmi, a White Diamond loathing Sooah for being given what she desires since her childhood: the hand in marriage of one of the prestigious Black Diamonds, Kim Jongin and Oh Sehun.

Struggling with her new position and the bitter reality of being placed up high for the many watchful and judging eyes waiting for her to fall and break, Sooah finds herself cut off from everyone by an invisible border no one dares to cross. 

Alone and with no one to turn to but her two fiancés of the same social rank, Sooah realizes that wealth isn't a fortune alone, but also a stigma branding them as idolized outcasts. Engaged to not only one, but both of the Academy's princes, Sooah, too, is now part of an elusive circle of three and moved to live with her two withdrawn fiancés, two heirs that couldn't be more different.

Playful and dangerous, Kim Jongin is all sensual body, dark hair and eyes, tan skin and honey smooth words. His smiles are a secret to unveil, and in his eyes Sooah sees a bottomless darkness lurking that has her both enthralled and recoiling. 

Distant, yet oddly caring, Oh Sehun is the light to Jongin's darkness. Pale skin and hair, subtle expressions on his even face, he is of few words, but attentive eyes, with a gentle calm to his every touch. Behind the mask of a cold ice-prince, Sooah sees a glimpse of so much more whenever he quietly guides her through the difficulties she faces. 

While Sooah is initially confused about the lack of protest from her two suitors concerning the life-changing events, she gradually realizes that a marriage with her would result in a merge of two of the most powerful houses in Asia, a duty both heirs grew up eager to fulfill, while Sooah only reluctantly agrees with her new role. 

Living together with two Black Diamonds she used to think of as princes and starting to mend her broken heart with their help, she realizes that while she is forced to decide on one of them, the final decision of whom she is willing to marry is ultimately hers to make.

-It's a game of love. who is it going to be?-