Father Brown - Season 6 Episode 10

Father Brown reads that Flambeau has been killed in Italy before setting off to Gloucester where the Iron Crown of Lombardy, made from a nail from the Crucifixion, is housed. He is approached by a woman claiming to be Flambeau's widow but a key Flambeau sent him has been stolen and Father Brown believes it is connected to the intended theft of the Iron Crown. A chance meeting on a train proves him right and he must stop the theft – at the risk of his life.