Waka Ninomiya and Miko Komine are two inseparable teammates, training hard with pure hearts like sisters. Their daily water polo practice in skimpy athletic swimwear is secretly watched by a perverted coach. He rarely shows up for drills, spending most of his time in meetings—but whenever he does appear, he finds an excuse to send one of them on an errand, leaving himself alone with the other. Innocent and unaware, the girls simply obey their coach’s instructions and encouragement. In reality, he exploits their naivety and lack of sexual knowledge, repeatedly forcing them into lewd acts under the guise of training. Before long, the girls begin to feel pleasure from these acts—but stay silent, unable to speak up. One day, while changing, they catch sight of their coach in the corner of the room, wearing only a tight competitive swimsuit. Furious at being spied on, they confront him—only to notice the obvious bulge in his crotch. Realizing his true nature, the once-innocent pair feel a sudden surge of sadistic desire. They overpower him, taunt him, grope his crotch, and aggressively dominate him sexually. This coach may have awakened something dark in the pure-hearted sisters—but should we condemn him… or thank him?