A missing plane, a daring plan and no time to lose!Peter and Elise Andersen and their friend Henrik Melchior are worried when Henrik's mother and a family friend, Matthias Karlsson, fail to return from a short flight to Sweden. Has something happened to Mattthias's plane?Then a mysterious caller informs Peter that Ruth Melchior and Matthias are being held by the Russian forces occupying the Danish island of Bornholm. The charge: spying! But the line goes dead before Peter can ask any questions.Frantic, Peter, Elise, and Henrik set sail for the island with Uncle Morten at the helm of the Anna Marie. But their arrival is met by secretive Russian soldiers who deny keeping any prisoners. How can Peter and the others persuade the Russians to release Mrs. Melchior and Matthias when the soldiers will not even admit they are there?