Guilty Angel

Yasuo is a shut-in living in the same apartment complex as three elementary school girls, whom he names "F3" (short for "Flower Three"). When he's not fantasizing about the girls, he enjoys observing and recording them from his window. One day, Yasuo sneaks into the girls' locker room at their school swimming pool and installs a hidden camera. Later, as he excitedly plays back the captured footage, what shows up on the screen is something he never expected to see.

(Source: Natalie Comic)