Whisked out of Paris ahead of the Prussian army in 1870, twelve-year-old Irene Adler is reunited in London with her friends, Sherlock Holmes and Arsène Lupin--but Arsène's father has been arrested for murder and the kidnapping of a famous opera singer, and it is up to the three young detectives to solve the mystery.

In March 1871 Irene's family moves from London to Evreux in Normandy, but after a strange woman warns her that her mother is in danger, Irene calls upon her friends Ars�ene Lupin and Sherlock Holmes for help and soon the three young detectives are caught up in the search for an ancient relic said to be in a secret crypt beneath the streets of Paris--a Paris which is torn apart by war and currently ruled by the Commune.