Father Brown prepares to go admire in Gloucester episcopal palace the Crown of Lombardy, a priceless relic from Italy, where master thief Hercule Flambeau reportedly died in an accident. He gets a visit from Lisandra Flambeau, Hercule's black widow, who seeks in vain a key Hercule sent the priest, but is stolen at the rectory. On the train to Gloucester, as Brown feared he meets very alive Hercule, he demands his help to steal the relic at ransom for Lisandra, who was kidnapped by her Corsican mob father. It's the perilous start of serial double-crossings.
—KGF Vissers