Shinichi Kudo, a Teitan High School student always helps police to solve difficult case, so he called as "The Savior of Japanese Police".
One day, after he solve a case called "Roller Coaster Murderer Case" he see men in black that he saw at the case. Because he wants to know what are they doing, he go following them. Then he see a transaction between a man with one of men in black he saw. But, another man in black saw Shinichi, and he hits Shinichi's head. And then, he gives Shinichi a newest poison that created by that organization. Soon, Shinichi became small again. He looks like a 6 years old kid.
Then he go to Professor Agasa's house (his neighbor) to get a help. Then he changed his name to "Conan Edogawa" to hides his identity. And he stay with Ran (his childhood friend) because her father (Kogoro) is a detective. So he can find any news/case related to those men that makes his body small and get the poison to Professor Agasa to learn it and make an antidote to make his body return again.