Sara, a modern-day mermaid, has long worked as an “exchanger”, handling trades between humans and yokai. To her, humans are nothing more than clients—never someone to trust, let alone let into her heart. One day, however, a mysterious and beautiful young man named Sora wanders into her secluded little sanctuary. He doesn’t seem like an enemy, but he clearly carries secrets of his own. Curious yet cautious, Sara offers him a deal. But the only thing Sora requests from her… is Sara herself. Thus begins a romantic and secret-filled cohabitation, born from a “transaction” neither of them expected.