From Renta!:

The one that the demon lord fell in love with... is a human! The human is such a crybaby that his nickname is Niagara; he goes to a hospital deep in the mountains to try and fix his temperament. Overwhelmed by the genius doctor, who's looks are so beyond beautiful that that he seems almost demonic... Niagara continues with his treatment. The kindness that the doctor sometimes shows him makes his heart pitter patter, but then, all of a sudden, he's pushed down and forced to undergo erotic shock treatment! Sometimes forceful, sometimes gentle, sometimes rough... the demon lord's treatment builds to a climax!