Welcome to the fascinating world of Gensokyo, in which youkai (A term for all sorts of spiritual creatures), animals and some humans live, completely separated from our world by a magical barrier. The only gateway between the two worlds is the Hakurei shrine. Reimu Hakurei is the maiden working at said shrine. Whenever problems of supernatural causes (known as "Incidents") occur, Reimu goes out with her friends to investigate, eliminate the cause and hopefully restore Gensokyo to its equilibrium.

High school student Taiyou Asano has been socially inept ever since his family died in a car crash. The only person he can properly interact with is his childhood friend, Mutsumi Yozakura—the head of the world's strongest family of spies. Mutsumi's eldest brother, Kyouichirou Yozakura, has been overprotective of her ever since he once rendered her severely injured. His love for Mutsumi is lethal, and Taiyou is his next target. To survive, Taiyou must marry Mutsumi and become a member of the Yozakura family. Thrown headfirst into chaos, Taiyou begins his journey to become a powerful spy in order to protect his wife and uncover the dark secrets of his past and the Yozakura family.

The entire universe is divided into the inner universe and the outer universe. The two universes are enemies of each other. The outer universe is ruled by demons, and the inner universe is divided into The Realm of gods, the Eternal Realm, and the Mortal Realm. In the universe, there are countless mortal worlds like the Tianfa Continent, and they are collectively referred to as the Jiutian Xin Region. In the field of Jiutian Xin, nine immortal emperors commanded all star fields in nine layers. Above the nine heavens is the realm of purification of immortal gods.

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