A group of Royal Marines are launched into the sea off the French coast from a submarine during WW2. They paddle up a river for three nights, hiding by day among riverside reeds. After misfortune to one canoe, and a meeting with the French Resistance fighters, the others reach the port and place limpet mines on enemy ships berthed alongside the quays. They make their escape, suffering further mishaps. The story continues, following one marine's endeavours to evade the enemy, with flashbacks to the events that happen to the others.