Shiina Natsukawa

The story takes place in an age where "Ajin" (demi-human), more casually known as "Demi," have slowly started to become accepted into human society. Tetsuo Takahashi is a biology teacher who ends up teaching three such Demi, hoping to understand more about them while also managing to catch their attention.

With Japan having developed as a maritime nation, it became necessary to have specialized schools to teach skills essential to protecting the sea. The setting is one such all-girls school. The girls who enroll here study to become "blue mermaids," through classroom instruction and teaching of practical skills. "Blue mermaids" are women who pursue the ocean for their professional occupation. They live by the motto "In the sea we live, the sea we protect, and by the sea we go." They must excel in their studies, but must also adopt an international mindset. They are the "Elite of the Sea," and are expected to be knowledgeable of the customs and cultures of other countries as well as their own.

Takamiya Honoka is a regular student whose only problem seems to be that he sits next to Kagari Ayaka, the school's #1 beauty. They have never spoken to each other before and any small interaction between them immediately results in her fanclub beating him. Yet when a falling part of the school's building is about to send him to the afterlife, it's Kagari that comes to his rescue. Only... she's dressed as a witch, carrying him in her arms and floating on a broom?! Kagari tells him it is her mission to protect him and that now she can finally protect him openly rather than undercover.

The story begins one winter day when the wish of a few girls was realized with a miracle, changing the landscape of a town. "In the skies above this town, a disc is always there."