After losing his entire family in a car crash, Taiyou Asano tries to distance himself from his childhood friend, Mutsumi Yozakura, but she insists she will always stay by his side. As a result, Taiyou becomes socially withdrawn and has difficulty making friends at school, despite his classmates' persistent efforts.
One day, the vice principal summons Taiyou to his office, but instead begins threatening his life. Taiyou is whisked away by a stranger and later wakes up to find Mutsumi, who introduces him to her siblings—a family of spies with superhuman abilities. In each generation, one ordinary human is born to become the family head. Mutsumi is that person, and the family's duty is to protect her with their lives.
The vice principal's true identity is Kyoichiro Yozakura, the eldest son of the family. Believing Taiyou poses a danger to Mutsumi's life, Kyoichiro forces them into marriage as the only way to keep both safe, since the family has a rule against killing within the Yozakura household. Though hesitant at first, Taiyou agrees to protect Mutsumi. Now, he must train as a Yozakura spy to fend off the constant threats against both their lives.