To earn money for her studies abroad, Maria had been dating S clients, but one day she lost her temper and slapped her client. However, having majored in psychology, she didn't miss his reaction and quickly turned the tables. Recognizing his sadistic tendencies, she began treating him as a subject for her research. Under her control, she alternately tenderly and intensely stimulated his anus, making him moan helplessly as he gradually revealed his latent masochistic nature. Observing his psychological transformation throughout the process, Maria found increasing academic value in her experiment.