"Tantei Opera Milky Holmes: Arsene Karei Naru Yokubō" is a spin-off of the "Tantei Opera Milky Holmes" series, focusing on the character Arsène, a master thief with a flair for the dramatic. The story is set in a world where individuals possess supernatural abilities known as "Toys," which grant them unique powers. Arsène, known for his elegance and cunning, operates as a phantom thief, stealing valuable artifacts while leaving behind his signature calling card. His actions often bring him into conflict with the detectives of the Milky Holmes agency, who are tasked with stopping him.

The narrative centers on Arsène's elaborate heists and his interactions with the detectives, particularly the members of Milky Holmes: Sherlock Shellingford, Nero Yuzurizaki, Hercule Barton, and Cordelia Glauca. Each member of Milky Holmes has their own Toy ability, which they use to solve mysteries and combat crime. However, Arsène's intelligence and resourcefulness make him a formidable opponent, often outsmarting the detectives at every turn. The series explores the cat-and-mouse dynamic between Arsène and Milky Holmes, blending elements of mystery, comedy, and action.

Arsène's character is portrayed as a charismatic and enigmatic figure, driven by a desire to challenge societal norms and expose the flaws in the system. His heists are not merely for personal gain but are often motivated by a deeper sense of justice, targeting corrupt individuals and institutions. This moral ambiguity adds complexity to his character, making him both an antagonist and an anti-hero. The detectives of Milky Holmes, while determined to capture Arsène, also find themselves questioning their own methods and beliefs as they confront his unconventional approach to justice.

The series maintains a lighthearted tone, with comedic moments and exaggerated character interactions, while also delving into themes of morality, justice, and the nature of power. The interplay between Arsène and Milky Holmes drives the plot forward, with each episode featuring new challenges and revelations. The dynamic between the characters, combined with the series' blend of humor and intrigue, creates an engaging narrative that appeals to fans of both mystery and comedy genres.