The Last Redoubt — George Goodchild

The kidnapping of a young doctor leads to some puzzling developments, which have their origin in a small island in the Adriatic. The doctor's sister, Valerie, tormented by the disappearance of her brother, decides to act independently of the Police. She and Inspector McLean eventually find themselves marooned on The Isle of Mystery, at the mercy of a gang of criminals - but an inspired strategem on the part of McLean turns the tables completely.