Koko and his friends are spending summer vacation on the shores of a 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 cover of night, burglars break into one house after another, stealing anything valuable they can find. When the police turns a blind eye to denunciations by adults, the children decide to take matters into their own hands and unmask the burglars.