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.

On a starry night, middle-schooler Manta meets Yoh Asakura, a young shaman who has the ability to talk to spirits, and also has the ability to let them possess him to use their powers. Yoh has a heavy duty on his shoulders: he must become the Shaman King by winning the Shaman Fight, a tournament between shamans that happens only once every 500 years. Joined by his spirit medium fiancée Anna Kyōyama, his personal samurai spirit Amidamaru, and his other friends, Yoh must also face a larger evil: Hao, a powerful man who aims to create a new world with only shamans by becoming the Shaman King himself.