Case Closed

From Adult Swim Headquarters Wiki

The series Case Closed, called "Detective Conan" in Japan, revolves around Jimmy Kudo. Kudo is a teenage detective known for his excellent sleuthing skills. Unfortunetly, Kudo is attacked while on a case and given a drug which turns him back into a boy. He becomes obsessed with finding out who did this to him, but is forced to take on the name Conan Edegawa until he finds him.