Nature Versus Nurture

Cha Seon-Ho lives a humble yet diligent life, but something has been bothering him for months: someone keeps leaving a sticky mess in his locker. As the cruel pranks escalate, Seon-Ho and his best friend, Lee Yu-Geon, decide to catch the culprit by staking out the locker overnight. But when Seon-Ho takes a break to visit the restroom, he discovers something deeply unsettling has happened to his own body…

(Source: Lezhin US, edited)