[NASK-215] In fact, I was totally into my mother, but I was alone in a mixed bath... I let my guard down because she was just a play, and when I saw her naked body, I got a rock hard erection. This is the whole story of the mistake I made. (Censored)

A hot spring trip for mother and son. When they went to the mixed bath for the first time in a while, the son saw that his body had become that of a fine adult man. The son also got an erection when he saw his mother's naked body, which was still beautiful and firm. Where will the two end up...? This is the fourth installment of the series!