Brittany, France, 1941 - Annette Renard leads a double life; a teacher by day and a French Resistance fighter by night. Her group's exploits include attacking Nazi freight trains, battling enemy patrols, and helping shot-down bomber crews escape to England. But Marcel, the group's leader, has his sights set on something bigger - destroying a large munitions dump by tunneling under its defences. Unexpected parallel events cause navy lieutenant Jimmy Wainwright, a Motor Torpedo Boat captain, to become a participant in the attack. However, there may be a traitor in the group who could cause everyone's death.