Complete Breeding: The Aesthetics of Confinement

Eun Hee and Jung Yeon head toward a pension to have a blast with the guys. Chul Soon is the janitor, and he is caught sneaking a peek at Eun Hee and Jung Yeon tanning but gets away with it in return for letting them take an embarrassing video of him. That night, while having dinner, Eun Hee excuses herself from the table, and Jung Yeon and Min Ki have sex. They realize someone is watching and thinking it's Chul Soon again, beat him up. However, Chul Soon is innocent, and he decides to take revenge.
(Source: HanCinema, DramaWiki)