It all starts beneath a treadmill with a suspicious stain that definitely does not belong there. Yonggeun Fitness is one of the most popular personal training gyms in Seongwang-gu District, and its night class in particular is so sought after that male members compete fiercely to get in. When the building’s owner, Seon Bin, discovers a strange residue left behind after one of these late night sessions, he begins to investigate what really goes on after hours in Seongwang-gu. What he uncovers is far from ordinary training. The so called night class turns out to be a secret hotspot where workouts are merely a cover for something far more indecent. Furious that such shameless behavior is happening under his roof, Seon Bin vows to put an end to it no matter what it takes.