Sherlock Holmes, Arsène Lupin, and Irene Adler meet for the first time in Saint-Malo in the summer of 1870. They were all supposed to be on vacation, but fate had other plans for them. The children find themselves entangled in a criminal whirlpool: a diamond necklace disappears, a dead man is found on the beach, and a dark figure appears and disappears on the rooftops of the city. The police are searching in the dark. The three friends take it upon themselves to solve the case.