Case Closed, Vol. 3 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. Jimmy, Rachel and Richard take a vacation aboard a cruise ship, but little do they know that the patriarch of the wealthy Hatamoto family is about to be murdered. With the perpetrator still aboard ship can you figure out whodunit before Conan does!?