In the tightly guarded city of Bardium, where magic, miracles, and illusions are strictly forbidden, a young man named Norman finds himself tied to a chair in a dimly lit room, facing a frantic interrogation. His questioner, an overly zealous researcher, demands to know about the Unlaw, supernatural monsters who gain their abilities from unstable emotions. Four of these beings, all taking the form of beautiful women, are currently hunting one another, and Norman has been tasked with sating their desires while dealing with the chaos erupting across the city.

The narrative unfolds as Norman recounts the events of the past month, revealing his role as a detective and confidant to the four Unlaw women. He works with Shizuku, a stoic musician, to solve a gruesome society murder. With Eltiel, a huge and fiercely loyal blonde woman, he investigates a depressing string of killings in the slums. He partners with Lonsday, a brilliant but easily bored detective, to track down a phantom thief stealing from a museum. Finally, he follows Clareth, a schoolgirl, into a trap involving a train car filled with corpses. As Norman assists each woman, it becomes clear that his affection for them is genuine; he loves them not as dangerous Unlaw, but as human beings. Despite the grim setting and high body count of the crimes surrounding them, the core group is defined by their surprisingly sweet dynamic.

The first volume concludes with a climactic battle that reveals Norman is far from the passive protagonist he initially appeared to be, showcasing his willingness to kill in order to protect his loved ones. Following the incident, the four Unlaw women and Norman are rewarded by opening the Elementary Detective Agency, setting the stage for their future, albeit forced, interactions.

A month after these events, Norman and his four Unlaw companions begin to settle into their new lives at the detective agency, though the city of Bardium remains as dangerous as ever. In the second volume, Lonsday and Eltiel team up to take down a criminal known as the Mad Bomber, a case that leads them to discover the existence of weapons designed to fight the Unlaw. Simultaneously, Clareth and Shizuku head to Evergrace Academy to investigate the mystery of a murderous Banshee. As the dust settles, the group comes to realize that these seemingly separate cases are more deeply connected than they initially assumed.