While on summer vacation at the seaside, twelve-year-old Irene Adler meets the young Sherlock Holmes, and his friend Arsène Lupin--and when a dead body floats ashore the three young friends set out to solve the mystery.
Receiving a warning that her mother is in danger from a woman who promptly vanishes, Irene, Sherlock, and Lupin follow clues to a secret crypt beneath the streets of Paris, where they discover a priceless relic.