The adventures of the inseparable six young friends from Croix-Rousse and their faithful dog, beloved by readers, continue. Every time the kids go on vacation, they inevitably end up in some mysterious and dangerous story. Now the friends will encounter a mysterious foreigner, suspicious woodcutters and cunning jewel box thieves. What kind of strange sailor is lurking in the nooks and crannies of the ship on which five friends and their faithful dog Kafi are traveling? What message is he transmitting on the radio and to whom? Why did he need to do this in secret? And is he involved in the daring theft of diamonds from one of the passengers? All these questions could have remained unanswered for the young detectives if not for Kafi and... a tiny inflatable boat.