Father Brown - Season 3 Episode 4

Father Brown and Lady Felicia visit the local barracks for the unveiling of the Dunkirk war memorial for which they have helped raise funds. The hero of Dunkirk was Colonel St Clare, who allegedly killed several Nazis with his broken sword but at dinner drunken Major Rawlings accuses St Clare of cowardice and is later found slain with the sword. St Clare is the obvious suspect but Father Brown, believing him to be innocent, discovers another person who remembers what happened at Dunkirk and has waited thirteen years before taking action.