One hundred Mamodo (lit. "demon") go to Earth every 1,000 years to battle to be the king of the Mamodo world. Each Mamodo needs a human partner in order to use his or her spell book, a book that seals the powers of the Mamodo. If the spell book is burned, the Mamodo is forced to return to the Mamodo world. The last Mamodo standing without their book burnt is the new Mamodo king.
Yusuke Urameshi, 14, is the worst student of his class. He's always in fights and seems to always be the bad apple. Then one day it all changed. He died saving a little boy from traffic. Because of this self-less act he goes through an ordeal to get his body back...and he becomes a spirit detective. As spirit detective Yusuke goes on missions for the spirit world to eliminate demons and save the human world.