It seems the sealing of the Demon King has failed...
The Demon King, who commits all kinds of atrocities without regard for his country, was sealed away in time with the moment when
the Demon King was at her most relaxed, during a ritual, when she was at her climax. The Demon King's body, which was meant to be an empty vessel, somehow housed the consciousness of a young boy... (What... my body is shaking?) The boy, Ritsu, who entered the Demon King's body in the middle of the act, was at the mercy of unexpected pleasure. Unable to control the magical power that resides in the Demon King's body, Ritsu accepts a magical tool that seals and absorbs the magical power. The magical tool influences Ritsu's soul, and his body becomes younger. The rejuvenated Demon King's magical power becomes stronger the more pleasure he feels, and the King's consort and Prime Minister notice this and decide to give the Demon King an "education"... Ritsu endures days of forced pleasure , believing that he will one day return to his original body and return home . The hero who came to defeat this weak demon king turns out to be a demon king sealed in Ritsu's body in another world...