Bae Woong used to think of himself as a robust and athletic man. Desperate to recapture his former physique, he begins searching for a local gym. He finds one that appears unusually costly and secretive. Intriguingly, details about their workout routines are not disclosed until after he signs a nondisclosure agreement. Though perplexed, the allure of the attractive staff members convinces him to go forward; after all, there's nothing to lose. Upon signing the agreement, he uncovers the secret behind their enviable bodies: it revolves around sexual activity.
Bae Woong had always seen himself as a fit and energetic individual. Determined to get back into shape, he starts looking for a gym in his area. He comes across one that seems unusually high-priced and enigmatic. Curiously, specifics about their training methods are kept under wraps until after he signs a nondisclosure agreement. Even though he is puzzled, the attractiveness of the staff sways him to proceed; after all, what harm can it do? Once the agreement is signed, he discovers the secret to their remarkable physiques: it centers on sexual activity.
Maybe the author just uses the old concept of time travel. No more no less