It is Saturday evening and Father Brown finds a note on the mat which says 'to catch a thief, look to the cross'.
After checking to see if the Blue Cross, an ancient holy relic, is still in the church, he then discovers a dropped handkerchief with a monogrammed letter ?F'...
Father Brown is based on G. K. Chesterton's detective stories about a Catholic priest who doubles as an amateur detective in order to try and solve mysteries.
Father Brown - The Blue Cross Synopsis and Schedule