Every one thousand years, one hundred mamodos (demons) descend upon earth to conduct the ultimate battle. The winning mamodo becomes the mighty king of the mamodo world. There is only one problem - in order for the mamodo to use their powerful spell books, they need a human partner. If his spell book is destroyed, the mamodo disintegrates and returns to the mamodo world.
Kiyo is a brilliant but aloof 14-year-old boy in junior high school. While exploring a rainforest in England, Kiyo's archaeologist father discovers Zatch, an unconscious mamodo child. He sends the goodhearted and mysterious Zatch to be his son's mentor. Zatch has amnesia; he does not have any idea that he is indeed a powerful mamodo! It is up to Kiyo and Zatch to discover this together.
The unsuspecting Zatch and Kiyo must defend themselves against the relentless ambush of mamodo-king wannabees. And unbeknownst to the attackers, Zatch and Kiyo have formed a special bond which enables them to unleash Zatch's incredible powers.
Season 1 Zatch and Kiyo are new to the mamodo battle and they learn that there is a battle every one thousand years and learn that from Sherry and Brago and along the way they face against strong mamodo and also befreind them on there way to becoming the next mamodo king.
Season 2 Milordo Z Zofis creates an army of ancient mamodo from a thousand years ago and Zatch and Kiyo and there friends must travel to the Devolo Ruins in South America planed by Dr. Riddles and beat them all and take down Zofis who is manipulating all those humans and mamodoes and along the way the gang befriends a girl named Laila who is a mamodo from a thousand years ago and her book keeper Albert who she wants to wake up by destroying the moonlight stone. Will they ever defeat Zofis or will he defeat him.