Sara, a mermaid living in the modern world, has been dealing with yokai and humans alike as a "Trader." To her, humans are merely clients—trusting them with her heart was never an option...

One day, a mysterious young man named Sora wanders into her miniature garden. He doesn't seem like an enemy, but there's clearly something unusual about him. Sara offers him a deal. However, the one thing Sora truly desires turns out to be none other than Sara herself...!?

A cohabitation romance filled with love and secrets begins—all starting from a single "deal"...