"IF ODD, CHANGE. IF EVEN, REMAIN THE SAME..."

In the marquess of Nan'an's household, Lu Wancheng's death has left Lin Qingyu a young widow, and he loses no time fleeing his late husband's hostile family to continue pursuing his dashed dream of becoming a physician. Indeed, the empire of Great Yu has never been as desperate for fine physicians--both the emperor and one of his finest generals are in such ill health as to threaten the country itself.

Lin Qingyu's medical talents are far from rusty, and soon enough, they ensure that he's ushered into the imperial palace. There, he waits for someone to utter the secret code Lu Wancheng taught him--the code that begins "If odd, change. If even, remain the same..."

When Lin Qingyu finally hears those words, the speaker isn't the person he expected--nor are they the only person in the palace interested in Lin Qingyu. The young physician's beauty and talent have created professional rivals and even more serious threats. The speaker of the secret code may have come too late.