To be honest, my sister-in-law is plain and unattractive. I was secretly observing her for fun, and to my surprise, underneath her tacky loungewear she was wearing sexy underwear, always ready for action! Huh? Why? It turns out my sister-in-law's true nature is that of an aggressive slut who goes out hunting for men whenever her sexual desire builds up! But that day, she couldn't find a partner, so she exploded with frustration and attacked me! I was fucked to my heart's content by this super-slutty sister-in-law, a woman I'd never known before... I never imagined that my plain sister-in-law could be so erotic... Honestly, I underestimated her. I'm sorry.