Ran Mōri and her friend Sonoko Suzuki are invited by Wakamatsu, one of the two guys they had met the other day, to join him, and a couple of friends a valentines day party, to which they agreed to go to. They arrive as well as Conan, who sneeked on to the premises as well as getting caught there. Tragedy soon strikes the party as Katsuhiko lies on the ground dead from poison. Naomichi had originally confessed to killing Katsuhiko, but Conan figures out he is not the true murder, but is instead Katsuhikio's own mother. The mother eventually admits to the crime, needing the life insurance money on Katsuhiko's name, and is put away.

Kudo Shinichi is a seventeen year-old high school detective whom people call the "Modern Sherlock Holmes." However, one night after a date with his childhood sweetheart, Ran, Shinichi witnessed an illegal trade and, caught off his guard, was knocked unconscious and fed a drug that was supposed to kill him... but he woke up and found himself shrunken to a seven year-old. In order to track down the men who did this to him, Shinichi hid his identity and lived with Ran, whose father happened to be a hopeless detective, and with that came a series of murders and mysteries that he must solve.