Season 1, Episode 10: Season Finale

Father Brown receives a mysterious note suggesting that the church's priceless holy relic, the Blue Cross, will be stolen. He later finds a handkerchief with the letter F, no doubt left there by the infamous thief Flambeau -- wanted across Europe for stealing valuable artifacts.

Bishop Talbot appears on the scene and makes arrangements to have the cross transported by police to safety. But Father Brown decides it would be better off in his possession. He ignores Valentine's advice and sets off on his own to Newburgh Priory. He is out not only to identify the thief, but to also save his soul.