Koko and his friends are spending their summer vacation by the lake near the idyllic Green Hill, hanging out and playing. The magical countryside is disturbed by unusual events which alarm the children as well as the locals: under the cover of night, burglars break into one house after another, stealing anything valuable they can find. When they poison Koko’s and Marica’s dog, and the police turns a blind eye to denunciations by adults, the children decide to take matters into their own hands and unmask the burglars.