The Mysterious Boy is a film about a group of 14-year-olds solving a mystery and at the same time discovering what it means to be adult. Koko’s friend Tomo recently came to town and is starving for adventure. In school, the new, mysterious, funnily-behaving boy attracts the boys’ attention. Together with their school mate Marijana they embark on an adventure that will finally reveal the new student’s well-kept secret. Still, solving mysteries is not a game and the resolution is much scarier than they first thought.