Koko and his friends are spending summer holidays on the lake near the idyllic Green Hill, hanging out and playing games. The harmonious countryside bliss is disturbed by strange events that stir commotion among both the children and the local population: a gang of robbers starts breaking into houses every night, taking away all of value. When the police ignores the grown-ups’ reports, the children decide to take matters into their own hands and catch the thieves. The fourth sequel of the Croatian hit serial based on the popular novels by Ivan Kušan is rather a prequel to the previous films (Koko and the Ghosts, The Mysterious Boy, Love or Death).