Shizuka Kamishiro is 21 years old, with no money, no friends, and no girlfriend. He was stuck at home, after he got scammed by a college acquaintance who wasn't even really a friend, when he heard a loud gasp from his neighbor, a college girl who was fucking her boyfriend while he's lying on his mattress suffering all alone. The absurd difference in circumstances drove him crazy, and he punched the wall so hard that he broke through. The neighbors came over, and her boyfriend beat him almost senseless. As despair and dark emotions overwhelmed him, a messenger from the demon world appeared. Immediately, the boyfriend's head flew off, and the demon told him, "So, go ahead and XXX this girl."

(Source: Futabasha, translated)