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...

人類が衰退し始めて数世紀、人は突如現れた「竜」という存在に、その住処を奪われ続けていた。

「誓約」と呼ばれる儀式で竜に人の領分への不可侵を願い出る機関・誓約課に所属する斎と草乃。相棒の竜・ハチと共に全国を駆け回り、竜と人との調停役を務める彼らが出会う、雄大で無慈悲な竜の姿と、竜の傍らで懸命に、時に狡猾に生きる人々の姿とは――。