When Father Brown reads about Hercule Flambeau's untimely death in Italy, he revisits a cryptic letter and key recently sent to him by Flambeau. Meanwhile, The Iron Crown of Lombardy, a crown holding a nail that was used in the crucifixion of Christ, arrives at Gloucester Cathedral. After Mass, Father Brown is greeted by Lisandra, who reveals herself to be Flambeau's new wife. She recounts the day that he died and reveals that she knows about the letter and the key. On returning to the presbytery, Bunty and Mrs McCarthy find it in a mess, the key stolen. Considering Flambeau's letter, Father Brown realises that the key could open the safe which holds The Iron Crown. Eager to inform the Bishop, Father Brown gets the train to Gloucester and is unsurprised to find his old frenemy alive and well waiting for him onboard. Can Father Brown manage to protect the crown and redeem Flambeau once and for all?