Married woman Miyu Morita is told by her loving husband that she might have gained a bit of weight, prompting her to decide on a diet. Aiming to lose weight healthily, she begins training at a gym under the guidance of a personal trainer. However, the trainer's behavior gradually becomes more aggressive, and his persistent touching slowly overwhelms her senses. Before she realizes it, her breasts are fully exposed, surrounded by the trainer who starts jerking off right in front of her. Watching him, Miyu unexpectedly feels arousal stirring within her. Though shocked by his sudden actions, the moment he yanks down her panties and thrusts into her from behind, she moans out loud from pleasure. Torn between guilt over betraying her husband and the intense pleasure she feels, Miyu is drawn deeper into an affair with the trainer. This illicit relationship becomes a life-changing event that profoundly shakes her heart.