Sherlock Holmes, Arsène Lupin, and Irene Adler meet for the first time in Saint-Malo during the summer of 1870. They are all on vacation, but fate has other plans for them. The children find themselves caught in a criminal whirlwind: a diamond necklace goes missing, a dead man is discovered on the beach, and a dark figure appears and disappears on the rooftops of the city. The police search in the dark. The three friends take it upon themselves to solve the case.