In the story, the Great Era of Detectives — when detectives and thieves with a power known as "Toys" compete — has given way to the Great Era of Idols. However, someone starts stealing the idols' songs in Yokohama. When super idol Marine Amagi's hit song "Kiseki no Uta" (Miracle's Song) is stolen, she enlists the detectives of Milky Holmes.

(Source: Anime News Network)