Yun Qianqian, an unloved princess of the grasslands, was supposed to be sent to marry into another kingdom in her elder sister's place. But after being tricked into taking her sister's place, she unexpectedly marries Pei Jin, the emperor of the Sheng Kingdom, who is rumored to be cruel and bloodthirsty. Pure-hearted and innocent, she knows nothing about the schemes and power struggles of the imperial harem and never tries to flatter anyone, treating others only with genuine sincerity. Beneath the palace's seemingly calm surface, dangerous undercurrents swirl, and schemes from all sides bring countless threats her way. Yet the cold and ruthless Pei Jin is deeply moved by her sincerity and purity. Again and again, he steps in to protect her, punishes those who plot against her, and tenderly treats her wounds. Everyone else sees her as nothing more than an unwanted pawn sent from the grasslands, and no one believes in her. But with her wholehearted sincerity, she gradually becomes the one irreplaceable cinnabar mark in the tyrant's heart.