Two items catch Father Brown's eye in the newspaper: Flambeau's death in Italy and the Iron Crown of Lombardy containing a nail from the True Cross on display at Gloucester cathedral. Father Brown opens a letter with a key that Flambeau had sent to him a few days before his death. A woman claiming to be his wife Lisandra Flambeau wants the key, but it is stolen while the presbytery is empty. Father Brown becomes part of an elaborate plan of Flambeau and his wife to steal the crown requiring a key and a combination to a safe. Flambeau double crosses his wife and to flush out her husband, she injects Father Brown with a slow-acting poison.