In the centuries since the human race began to decline, people have been deprived of their homes by "dragons" that have suddenly appeared. Sai and Kusano belong to the Oath Division, an organization that requests inviolability of human territory from dragons through a ritual called "oath". They travel around the country with their partner dragon Hachi, who acts as a mediator between dragons and humans. The majestic and merciless dragons they encounter, and the people who live beside the dragons, sometimes with hard work and sometimes with cunning...