When Father Brown hears that Flambeau, an international art thief, is planning to steal a priceless cross once owned by Saint Augustine during its transportation to Rome, he is delighted at the opportunity to match wits with a criminal of such repute.
However, Flambeau outwits Father Brown on their first encounter deep in the catacombs of Paris and vanishes with the relic. Now, the amateur sleuth must somehow lure the master criminal out of hiding, recover the cross – and save Flambeau’s immortal soul into the bargain…