One hundred Mamodo 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. When read aloud, the spells are cast by the Mamodo producing many effects. If the spell book is burned, the Mamodo is forced to return to the Mamodo world, and they lose all claim to the position of the Mamodo king. While the spells in each book typically are different for each Mamodo, there are Mamodo that share spells, like Zatch Bell and his evil look-alike, Zeno Bell. The human and their Mamodo gain these spells through experience and hard work. The last Mamodo standing without their book burnt is the new Mamodo king.

The story follows, Kiyo Takamine, is a 14-year old boy in junior high school. His father, Seitaro Takamine, discovers an unconscious child named Zatch Bell while in a forest in England who then sends Zatch to live with Kiyo. Unlike the other Mamodo, Zatch lost his memory of the Mamodo world. Kiyo learns about the spell book when he reads a spell causing Zatch to fire lightning from his mouth (the spell is mentioned in the first episode). As Kiyo and Zatch begin to encounter different Mamodos and learn more about the Mamodo battles, they discover that there are those who do not wish to fight and there are those who fight for the wrong reasons. After meeting a Mamodo named Kolulu and seeing how this kind Mamodo was forced to fight due to the power of her spells, Zatch decides to become a kind king in order to stop the battle from ever happening again. As the story progresses, Zatch and Kiyo meet other Mamodos that share similar views to them and become allies.