Kiyomaro Takamine is a brilliant junior high school student, so smart that he doesn’t fit in with the other kids in his class. One day, his father sends him a strange red book as a birthday gift, and tagging along with it is a mysterious young boy, Gash Bell. 
Gash Bell vows to get Kiyomaro back on track and help him get rid of his antisocial tendencies; sure enough, as Gash Bell helps him perform heroic deeds such as taking on the class bully, Kiyomaro slowly begins to change his outlook on life. One thing is for certain: Gash Bell is no ordinary kid. When Kiyomaro opens the red book and chants magic phrases, Gash Bell is able to unleash a variety of incredible powers! 
There is a reason for this: every one thousand years, a great battle is held to determine the next King of the Demon World. One hundred demon children are sent to the Human World where they partner up with human counterparts and battle until only one remains, the next King of the Demon World. And Gash Bell is one of these 100 demon children!
As other demon children competing for the throne approach Kiyomaro and Gash Bell to do battle, the two become closer and closer friends, and start to grow up and mature together. As the days of endless battle continue, Gash Bell comes face-to-face with a sweet young girl of the Demon World, Colulu. Although her fate lies in being returned to the Demon World after battling Gash Bell and Kiyomaro, her tenderness and strong will make Gash Bell determined that he will indeed become the King of the Demon World, a good-hearted King that would make Colulu proud.
Now, only questions remain: will Kiyomaro and Gash Bell be victorious against the other demon children? Will Gash Bell regain his lost memory of the Demon World, and will the secrets of his past be revealed? The answers lie in the grand adventures of Gash Bell!!