Najica Hiiragi is a noted perfumer for CRI Cosmetics, famous for being able to identify over 500 scents. As the series begins, she is struggling to finish "The Day Series" (a series of scents modeled after the days of the week), trying to find the perfect scent for the final perfume, "Sunday".

Secretly however, she is a field agent for CRI's Intelligence Bureau, an agency unknown to the public (and even to most of its own employees for the sake of their own protection). 

One particular assignment she is given by the organization is to rescue a girl caught on a small island. Though she safely rescues the girl, the organization has ordered Najica to go into a partnership with the girl. Reluctantly, she starts living together with the girl. But, this girl has a certain secret....