Kei Nagai dies in a car accident, but realises he is a type of immortal known as Ajin. Hunted by humans, Ajin is eventually found by the government and used as a subject in cruel experiments. He finds fellow demi-humans along the way as he escapes and goes on the run.

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。

Interpol Inspector Zenigata begins a world-wide hunt for the infamous thief Lupin The 3rd. But together, they face a powerful enemy and form the Lupin team in response. The Lupin team then faces the challenge of retrieving the "Crimson Heart of Cleopatra" locked away in a fort like safe. The treasure is thought to allow its owner to control the world.

In 1868, after the Bakumatsu war ends, the ex-assassin Kenshin Himura traverses Japan with an inverted sword, to defend the needy without killing.