From the introduction:
Long ago Human and Beast digimon warred; but one day an angel appeared from on high, bringing with it a golden age of peace and prosperity. However, that peace was short lived as their savior would become a tyrant; yet when all seemed lost Ten Legendary Warriors rose from the masses, and dethroned this wicked ruler. Though the Ten Legendary Warriors vanished soon after their victory; they left three mighty and noble angels to watch over the world in their stead. The noble Seraphimon to uphold law and order, the wise Cherubimon to preserve and spread its knowledge, and the gentle Ophanimon to foster love and life. Alas, the wheels of fate began to follow a familiar path; as Cherubimon grew to resent his fellow angels, believing them unable to understand the plight of Beast digimon. Though Seraphimon and Ophanimon tried to quell his fears, the once noble Sage soon fell from grace; his disquiet growing into a wild fire of madness that would threaten the digital world.