From MangaHelpers:
In a certain empire, the young emperor's workroom has a ceiling space that is full of secret agents from many of the empire's member countries. They all wear masks, but they get along as friends while they peacefully keep surveillance on the emperor.
One of these secret agents is a young woman, who is less than a year from her 18th birthday. She receives word from her home country that she is to be transferred, and that she will become some noble's lover. However, when she arrives home, she finds the situation is a little different. The emperor himself has come to fetch her, and he intends to make her his empress. To her surprise, he is actually someone she knows, a man who was sometimes among the masked secret agents! Now she will enter her new 'post,' and see if she can make it as a future empress!