Marin is happy enough living in her tenement housing complex with her friendly neighbours, but being an orphan and extremely poor, she often gets teased at school. Her life drastically changes one day when another world mysteriously appears in the sky. To stir things up even more, lethal monsters are constantly dropping from the world and attacking Marin. Luckily, she finds an amulet in a local shrine that unleashes a living weapon named Melan Blue whose mission it is to protect her from harm. As the two learn about and grow to accept each other, Marin struggles to discover why she is somehow at the bottom of all this.