Detective Conan is about a child called Conan Edogawa solving various cases. In fact Conan isn't a child but he is a high school student detective Shinichi Kudou, who is a great fan of Sherlock Holmes.
In an incident he was forced to take a experimental poisonous drug which made him shrink into a size of a child. To prevent his true identity being discovered he changed his name, moved into his friend Ran's home and went to primary school. Ran's home is actually a detective agency and her dad is a useless detective, hoping that living there will help to find the men from the organisation who made him into a child.
Using his deduction skills and gadgets made from his next door neighbour Professor Agasa, he continues to solves cases while he is also constantly looking for a way to change back to normal.