The series features Mondo Ooya and his girlfriend Rokuna Hiiragi, whose father invented a way to travel to Mon World (Roku Mon Sekai: the Six Gate World), where all sorts of magical creatures live. Together, they try to find six monster items which, when combined, could connect the Six Gate World with planet Earth for the better of both worlds. Rockna and Mondo form the Mon Colle Knights, and when chanting a phrase they can merge into monsters and control them into battle as well as aid with spells. Almost every episode, they battle Prince Eccentro (Count Collection in the Japanese version) and his two lackeys who are after the same thing as the Mon Colle Knights, except that they intend to use the items to dominate both worlds.