Living semi-cohabitation with my girlfriend Miwa, her older sister Yuki—a fitness trainer fleeing her cheating husband—moves in. At Miwa's suggestion, I start personal training with Yuki. The close physical contact and her athletic body lead to a constant erection, which Yuki notices. Instead of backing off, she escalates the training into seduction, leading to a secret, sweat-soaked affair right under Miwa's nose.