The Mysterious Boy is another one in the series of stories about a clever 14-year-old boy Koko and his friends who solve mysteries. Koko’s friend Tomo has recently moved to the city and he craves adventure. Tomo and Koko are drawn to a new student who behaves strangely. Together with their classroom friend Marijana, they reveal his well-kept secret one step at a time. Are Koko and his friends up to the task? Is the answer more frightening than they thought and how will they deal with it?