Brothers Kaj and Tommy are spending their summer vacation with their cousins Peter and Marianne, who are the same age, at Kullen in Skåne, while their parents go to Italy. The young people become involved in a tangled spy plot where it is not entirely clear who is on the good side and who is on the bad side. Espionage, mysterious light signals, and strange code words suddenly become part of everyday life, as do wild chases in mine tunnels, kidnappings, and nighttime break-ins. It will be a summer vacation they will never forget.