In a closed kingdom where technology and magic power are fused together by a guide from another world, only those with magic power from generation to generation belong to a privileged class, and the thread spun by that magic power is the source of power. The royalty and nobility have hair colors that symbolize their magical attributes, such as wind, water, fire, and electricity, while the commoners, who have no magical powers, are born with brown or black hair. In such a world of magical privilege, Loretta is a noble who possesses black hair as she was born from her mother's infidelity and is is. Raised in such a way that her existence is hidden from the world, the unfortunate Loretta is visited with a new destiny--! 黒髪令嬢〈不貞の子〉に求婚したのは王国一の天才魔道具士!? ロレッタは伯爵のもと、平民との不貞の証である黒髪の子として生まれてしまった。 王侯貴族は魔力をあらわす髪色となり、魔力を持たない平民は茶や黒髪で生まれてくる。 異界の導き手によるテクノロジーと魔力が融合する王国では魔力で紡がれた糸が動力源になり、その魔力を持つ者だけが特権階級に属する世界。 そこで隠されるように育てられたロレッタに、婚姻という新たな運命が訪れる──!!