During a transatlantic voyage, a young Englishman grows suspicious of two fellow passengers, Flannigan and Muller, who guard a strange little square box with unsettling secrecy. Convinced they intend to destroy the ship, he confides in a friend who doubts his fears. As their behavior becomes more cryptic, tension escalates-until the box's true purpose is revealed in an unexpected, ironic twist typical of Doyle's lighter tales.