September, 1940. German bombs fall on Britain, and enemy fighter planes blacken the sky around the Epping Forest home of Susan Shepherd and her grandfather, Bertie. The two raise homing pigeons, and Susan's favorite is Duchess. Hatched from an egg that Susan incubated in a bowl under her grandfather's desk lamp, Duchess shares a special bond with Susan. A Maine crop-duster pilot, Ollie Evans travels to Britain to join the Royal Air Force. In the National Pigeon Service, Susan is involved in an assignment code-named Source Columba, to air-drop hundreds of homing pigeons in German-occupied France, hopefully to convey crucial information on German troop movements. Friendship between Ollie and Susan deepens, but when Ollie's plane is downed behind enemy lines both know the chance of reunion is remote.