The Start of Kudo Shinichi's Investigation of the Black Organization.

Kudo Shinichi is a Junior at Teitan High School. He is extremely into detective novels, and his favorite detective is Sherlock Holmes because of his great powers of observation and deductive reasoning. Shinichi is the son of a world renowned mystery author Kudo Yusaku, and actress Kudo Yukiko (Fujimine Yukiko). 

He is a fan of soccer and plays in order to develop his physical coordination needed as a detective.

He has become somewhat famous as a high school detective who helps the Japanese police on difficult cases.

He is childhood friends with daughter of detective Mouri Kogoro and Lawyer Kisaki Eri, Mouri Ran (captain of the Karate club at school). Kudo promises Ran that he will take her to an Amusement park (Tropical Land) if she wins the city karate tournament.

While in line for a roller coaster Kudo displays his powers of deduction and it's hinted that Kudo has deeper feelings for ran than just friends. 

After they get on the roller coaster there is a murder, and Kudo helps the police find the killer. The suspects include the passengers on the roller coaster. That case is wrapped up, but as Kudo and Ran are leaving, Kudo notices on of the suspects who was on the roller coaster (who was with a partner, dressed all in black) acting suspiciously, so Kudo follows him and tells Ran to go on home.

It is revealed that he is meeting with some company president to collect money from black mail over that man's company's involvement in gun smuggling. The company president references that the man in black's syndicate commits much worse crimes than his company has.

While absorbed in watching this transaction, Kudo fails to notice the other man in black sneaking up behind him. He hits Kudo in the head with a pipe. They want to keep him quiet but can't use guns since the amusement park is still full of cops from the murder case, so they give him a poison that their syndicate is working on that is supposed to be undetectable in an autopsy, but it has never been used on humans.  

They leave Kudo to die. But, after Kudo looses consciousness for a short time, he awakens to some police who have discovered him. That's when it's revealed that Kudo's body has shrunken to a child.