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 Arsène 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.

In London, at Christmas time in 1870, a strange message in the classified ads leads the three young detectives to murders that appear to be connected with the Scarlet Rose Gang, whose leader was killed years before.