During the assassination attempt on King Alexander of Yugoslavia in Marseille, the king's valuable watch disappears. Herlock Sholmes is tasked with finding it. He quickly concludes that the watch was not lost but stolen by a pickpocket from Zagreb. He immediately travels to that city. On the Orient Express, he meets Arsène Lupin, who is also looking for the watch. From then on, a fair duel ensues between the two rivals. A matter of state? A matter of money? A former mistress of the King, opponents of the Yugoslav regime, a beautiful adventuress: so many adversaries who will allow Lupin and Holmes to measure their talents.