There are mysteries of all sizes in the world. When these mysteries are solved, they will release strong or weak energy. Brain Eater Naro (voiced by Takehito Koyasu) is a demon who specializes in eating these energies. When there are no more mysteries to be solved in the demon world, he came to the world just to find the ultimate mystery that can satisfy him. Katsuragi Miko (voiced by Kana Ueda) is a high school girl with a tragic past. Her father died, but the murderer has not been found yet. Fate brought Miko, who is full of mysteries, and Naro, who yearns for mysteries, together. With the help of Naro, Miko finally solved the truth of the incident. Afterwards, Naro and Miko opened the Demon Detective Agency. Although Miko is the honorary director, it is Naro, who has the power of a terrifying demon, who really runs the detective agency and solves the mystery. The two joined forces to solve a series of incredible cases, making the dark human nature nowhere to hide in broad daylight.