Homare Nakajima is renting a house on the cheap in a downtown Tokyo residential area but whenever he comes home he is often assailed by the unpleasant, wafting stench of sewage. One day, unable to bear the smell any longer, he sought out the source and managed to tear open the wall behind a cupboard. He discovered a hidden passage leading underground! At that moment, he blacked out. When he regained consciousness, he found himself having sex with a mysterious beautiful girl who called herself "Isaka". And so begins an obscene, erotic mystery surrounding Tokyo's extensive "underground world".