Season 1

It's the first day of the new school year and the Murakami kids, Yuko, Momoko 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!!" (コスモ・マジック! メタモルフォーゼ!!, Kosumo Majikku! Metamorufōze!!) she transforms into the Masked Belle Poitrine. Later she gets an ally in Poitrine Petite, in reality her little sister Momoko.

年明けの新学期。極普通の女子高生、村上ユウコは、忘れていた初詣を済まそうと立ち寄った近所の神社で、神様に出会う。そして、宇宙を絶望と暗黒から救った伝説の女神の名を持つ戦士、美少女仮面ポワトリンに変身する力を授けられ、持病の胃カタルの治療に出かけたい神様に代わって、ご町内（とついでに宇宙）の平和と安全を守るために戦う羽目になってしまう。 奇妙な犯罪者たちによる事件が次々と巻き起こっていくその度に、コスモブレスの力でポワトリンに変身し、ユウコは悪に立ち向かっていく。