Mirko's secret becomes an adventure of Koko, Tomo and Marijana.

Witty and resourceful fourteen-year-old Koko solves yet another mystery - aided by his friends. One of them, Tomo, has recently moved to the city and longs for adventure. The boys notice that their new classmate acts curiously. Step by step, joined by their classmate Marijana, they unveil the boy's dark secret.—Kinorama