It's the first day of the new school year and the Murakami kids, Yuko and Takuto, are excited to be heading back and catch up with their friends. While Takuto makes it to school, Yuko stops by a shrine that doesn’t get too many visitors these days to pray for the new school year. At the shrine Yuko accidentally breaks the bell and drops it on top of a kami's head. Because she is the only person that stopped by the shrine in a long time, he grants her the power to become a superheroine, based on another ancient deity. It is her job to protect her neighborhood from the threats it faces. With the transformation phrase "Cosmo Magic! Métamorphose!!" she transforms into the Masked Belle Poitrine. Later she gets an ally in Poitrine Petite, in reality her little sister Momoko.