Case Closed, Vol. 1 by Gosho Aoyama

[Viz] Can Detective Conan crack the case…while trapped in a kid’s body? When ace high school detective Jimmy Kudo is fed a mysterious substance by a pair of nefarious men in black—poof! He is physically transformed into a first grader. Until Jimmy can find a cure for his miniature malady, he takes on the pseudonym Conan Edogawa and continues to solve all the cases that come his way. Ghastly beheadings, bloody murders, and coldhearted child abductions-- Precocious high school student Jimmy Kudo uses his keen powers of observation and astute intuition to solve mysteries that have left law enforcement officials baffled. Hot on the trail of a suspect, Jimmy is accosted from behind and fed a strange chemical which physically transforms him into a grade schooler! Taking on the pseudonym Conan Edogawa, he attempts to track down the people who did this to him. But until he finds a cure for his bizarre condition, Jimmy continues to help the police solve their toughest cases.