The worst protagonist secretly takes upskirt photos of his younger sister to use as jerk-off material. Today, his younger sister is casually reading a manga. Taking advantage of the moment, he holds up his smartphone to get new material. "Now!" he presses the shutter—click!! He messed up. He forgot to put it on silent mode. His suspicious sister snatches the phone, and the moment she sees the screen, she flips out: "What the hell!? You're disgusting!!" It's over. His life is over. ...But then, for some reason, his sister smirks and presents an unexpected condition: "Then how about you show me you jerking off with my panties, big brother? If you do, I'll forgive you." And so the protagonist ends up having to show his super tsundere, lewd younger sister his masturbation.