Detective Conan Plot

Shinichi Kudo is a 17-year-old high school prodigy who frequently helps the police in solving cases. He was with Mouri Ran at Tropical Land, then he spotted and followed a suspicious person. During an investigation, he is attacked by two criminals, codenamed Gin and Vodka, from the syndicate called the Black Organization. They force him to ingest an experimental poison, APTX 4869, which is supposed to kill him, and leave him to die. A rare side-effect of the poison, however, transforms Kudo's body into that of a child instead of killing him.

After gaining consciousness, he confide to Professor Agasa and adopting the pseudonym Conan Edogawa, Kudo hides his identity to investigate the Black Organization. He intends to pretend to be a child until he can find enough evidence to apprehend the syndicate, after which he can then safely find and administer a cure for his current condition. Edogawa enrolls at Teitan elementary school and forms the Detective Boys with three other children in his class: Yumi Yoshida, Mitsuhiko Tsuburaya, and Genta Kojima. To further cover up his activities, he moves in with his childhood friend Mouri Ran, whose father, Kogoro Mouri, works as a private investigator. Kudo continues to solve criminal cases as Conan, but usually poses as Mouri Kogoro with the help of special gadgets invented by his neighbor and friend, Hiroshi Agasa.

Then, Shiho Miyano, a member of the Black Organization and creator of APTX 4869, tries to leave the syndicate after her sister's murder, but is held captive by them. She attempts to commit suicide by ingesting APTX 4869; however, like Kudo, she is transformed into a child. She manages to escape and enrolls in Conan's school under the pseudonym, Haibara Ai. She joins the Detective Boys and assists Conan in his investigations on the Black Organization. Conan's quest has led him to help the FBI to plant a CIA agent, Kir, inside the Black Organization as an undercover spy.

Then, Conan still have to continue his quest to pursue the Black organization.

Source: http://en.wikipedia.org/wiki/Case_Closed#Plot