The film recounts the adventures of Julian, Dick, Anne, George and Timmy the dog on their first holiday together at the seaside. In an old smugglers' cave, the children happen to overhear a radio message that refers to the abduction of George’s father Quentin. The professor has set up a laboratory on Kirrin Island where he’s doing research on a new energy generation method. Someone has obviously taken great interest in the valuable research results. But neither the police nor George’s mother Fanny believe the children. They are now on their own and discover a conspiracy that almost takes their breath away.

It focuses on two friends, one of whom pursues a relationship with a mysterious woman who part owns a guest house and who he has been dating online. Problems occur when guests at the isolated guest house begin to disappear and the guys discover the truth about the woman and her monstrous sisters and how they must escape from a mysterious island if they want to survive beyond dawn.