The anime is set in Bayron City, an artificial island city established, outside of any country's jurisdiction, in the Pacific Ocean to exploit local natural resources. In particular, the mining of the new energy resource orgonium has resulted in an outbreak of "D disasters" by demons in the city. Private military companies (PMCs) are tasked with dealing with these incidents.
The protagonist Shuu runs a small PMC, though his spending habits have left him constantly penniless. A demon girl named Kisara does contract work at Shuu's office and is constantly worried for him. Kisara attends a high school in Bayron City, and does everything from clerical work to household chores with confidence. Meanwhile, Ayano is Shuu's ex-girlfriend, an elite agent, and a former work colleague in a major PMC that Shuu used to work for.