Eiji Akutsu is a newspaper reporter. He is working on unsolved case that took place 30 years ago. The case involved a group of people extorting several companies for money. The group sent cassette tapes to the companies. Eiji Akutsu comes across a mysterious cassette tape. Meanwhile, Toshiya Sone lives in Kyoto and runs a tailor shop. He discovers a cassette tape among his late father's possessions. He plays the cassette tape and hears his voice from his childhood days.

本作品讲述了在警察“机动搜查队”，机动能力和运动神经首屈一指、但却没有刑事搜查经验也缺乏刑警常识的伊吹蓝（绫野刚 饰），观察和社交能力突出,但却不信任自己和别人的理性的刑警、志摩一未（星野源 饰），主角二人以24小时的时限内破案为目标的故事。 标题中的“MIU”是Mobile Investigative Unit（机动搜查队）的首字母，“404”是两人饰演机动搜查队员的呼叫信号。