From MangaHelpers:
The Kirionira are the demons that live in another dimension and invade the human world through their dreams. The only defense humanity has against the demonic assault are the hunters who rely on their sex drive as a source of power to fight against them. Riku Yoichi came from a long line of hunters and had high amounts of sexual energy which allowed him to be a great hunter. He moved away from his home town after middle school to demolish the Kirionira and was successful in finishing off the king and ending the battle forever. However, the peace came at the price of Yoichi losing his sex mojo. Returning back to his home town, he soon learns that the demon invasion was not over, but without his sex drive, he is helpless to fight back. He tries convincing his childhood friend Namba Midori and her female classmates to get him sexually aroused, but they want nothing to do with it. [vyc]