Shinichi Kudou is a gifted pupil specializing in detective work, known for his swift and effective resolution of complex cases. However, whilst keeping a watchful eye on two suspicious figures, he unwittingly stumbles upon an illegal activity that sets off alarm bells. In an attempt to silence Shinichi, the perpetrators administer a new drug they've created. To their surprise, Shinichi survives the assault but finds himself transformed into a seven-year-old child.