A twenty-year-old detective named Ritsuka Haruno dreams of capturing the great phantom thief Noir Rose, a criminal her own parents never managed to catch. Unfortunately, Ritsuka is clumsy, deeply afflicted with chuunibyou, and spends most of her time cosplaying as the protagonist of her favorite mystery novel, Madness Detective. Because her rank in the detective network is low, she only receives trivial jobs like searching for lost cats. To improve her standing, she needs an assistant and manages to recruit one by offering a daily supply of rice balls. That assistant is Ototo Saotome, a sixteen-year-old high school student with an enormous appetite, sharp physical instincts, and an exceptionally keen sense of smell.

The story follows this mismatched pair as they bumble through minor cases in an effort to raise their detective rank from the bottom of an alphabetical scale, hoping to eventually qualify for a job that could lead them to Noir Rose. Their small successes attract the attention of Hibiki Uruwashi, a wealthy rival detective from the neighboring office who builds impractical and frequently malfunctioning gadgets. As the duo gains experience, they encounter Jewel, a mysterious organization of phantom thieves whose members all use gemstone code names. The leader, Onyx, operates under the public identity of a detective network executive, while other members include a disguised woman named Citrine, a young inventor called Tourmaline, and Amethyst, who secretly works as Ototo's high school teacher. The narrative builds toward the revelation of Noir Rose, a female phantom thief positioned as a Robin Hood figure who only steals from villains and donates the proceeds to orphanages.