Kudou Shinichi is an exceptional high school detective. While investigating one, he was caught by a pair of nefarious men in black that he was watching and forced to take a dangerous experimental drug.
Leaving him for dead, the criminals disappear. Instead of being killed, the dangerous drug put him back to a little kid.
To preserve the illusion of his disappearance, Shinichi adopts a new name based on his favorite author (Arthur Conan Doyle) and Edogawa Ranpo and becomes Edogawa Conan. Ran, his childhood friend, takes this little boy under her wing and brings him to her home. Conan now lives at a detective agency run by Ran's father, as he relies on the possibility that he may cross roads with the men in black again. His mind is as keen as ever and continues to solve all the cases that come his way, but always allowing Ran's moronic father to take all the credit.
As far as everyone's concerned, Conan is just a little kid anyway, even to Ran. This is frustrating because nobody will listen to him.
In spite of this, he has a mission: to put the criminals who did this to him behind bars and get the antidote to that drug.