The cases of a detective whose physical age was chemically reversed to that of a prepubescent boy but must hide his true mental development.

Japanese high school student Kudo Shinichi is a well-known detective, known as the savior of the Japanese police. Once, when Kudo went out to play with his girlfriend Mulilan, he saw an illegal transaction going on. Kudo took his girlfriend to observe the deal, and suddenly a man behind him knocked him to the ground. The man and his accomplices let Kudo take a medicine. The medicine that would have killed him turned Kudo's body into the body of a primary school student! When Kudo woke up, he was surprised to find himself a child. To investigate the man and his accomplices, Kudo goes by the pseudonym Edogawa Conan (taken from the Japanese writer Edogawa Maimu and the British writer Conan Doyle).