This work tells the story of the two protagonists, Ibuki Ai (Ayano Go), who has the best mobility and motor nerves in the police "Mobile Investigation Unit", but has no criminal investigation experience and lacks common sense as a criminal policeman, and Shima Kazumi (Hoshino Gen), who is a rational criminal policeman with outstanding observation and social skills but does not trust himself or others. The two protagonists aim to solve the case within 24 hours. The "MIU" in the title is the initials of Mobile Investigation Unit, and "404" is the call signal of the two mobile investigation team members.