Mandie has just celebrated her thirteenth birthday and been surprised to discover that she has permission to go with her grandmother to Europe. It's just too exciting to be true! Grandmother Taft, Celia, and Mandie are given a big send-off as they board the Queen Victoria bound for Europe. Traveling on the ship with them is Senator Morton. His attentions keep Mrs. Taft distracted to the point where she slightly neglects Celia and Mandie a situation guaranteed to mean trouble. The girls meet some unusual people on board, and even find someone in their room when they return to it one night. Candy and fruit disappear from their cabin, and someone's valise turns up under their settee. Then comes the real surprise! Uncle Ned won't be able to rescue them this time!