Tsukasa Tenma is a seemingly normal, kind guy, but in fact he is interested in studying mythical creatures. One day he met a mermaid and for a long time could not believe it and decided to gain Rui's trust in order to study him later. So Tenma kidnapped a mermaid by deception and studied him, and then conducted non-lethal experiments and studied the body's reaction. He made a collar for Rui to lightly shock him for disobedience or other reasons