Cardinal Ratcliffe tasks Father Brown to find a priceless Papal tiara hidden in a 15th century castle. Father Brown takes Lady Felicia, visiting after a row with her husband, as a prospective bride looking for a wedding venue and Mrs McCarthy reluctantly as the brides mother. Flambeau appears at the castle having announced himself to the castle owner, Lady Cecily, as the groom wanting to surprise his fiancee. Flambeau declares he is a reformed man and his love for Lady Felicia. Circumstance dictate Father Brown and Flambeau work together to find the tiara when a criminal partner Flambeau wronged in the past turns up.
