A girl named Chiaki and and this older guy Ororon have been ostracized from their respective celestial realms. Chiaki because she is an impure angel and Ororon because he is the youngest son of the Lord Devil and therefore a threat to his older brothers for leadership of the underworld.
Most demons enjoy eating angels in this series and are after Chiaki. However the difference between angel and devil becomes clear when Chiaki has to convince Ororon not to kill those that come to threaten the both of them.

This mostly describes the first volume only. Other interesting incounters happen as the series progresses.