1942: an English arms factory. When the air-raid signal sounds, the workers go to the shelters, but foreman Fred decides instead to head for the roof to have a look. From the roof, he and the sentries watch as a German plane is shot down by a British fighter plane. After Fred leaves, the sentries discuss the part Fred's trip to France has played in Britain's success against enemy aircraft.
June 1940. As news breaks of the German advance in France, Fred becomes anxious that three 'special purpose' machines, on loan to a French factory in Bivry, may fall into enemy hands. He requests permission to go to France to retrieve the machines, but his manager is unconcerned. Determined, Fred tricks him into believing that the company's Managing Director has approved the trip.
After some bureaucratic delay in getting his papers, Fred leaves for France, where he boards a train for Bivry. When the train pauses in a nearby station, Fred heads for the café. While he eats his sandwich, however, the call is given to evacuate due to the approach of German troops. Fred, who speaks little French, doesn't understand the call, and is left in an empty station. He finds the station master, and tells him about his mission. The stationmaster pretends to telephone the factory at Bivry, but lies that the lines are down.
Helping himself to the stationmaster's bicycle, Fred makes his way to the factory, which he finds deserted save for an American girl, Ann. She is suspicious, until he presents his papers. As they ponder how to transport the machines, the local mayor appears, offering to take the machines to safety. He leaves, promising to return shortly with a truck. Ann tells Fred she believes the mayor is working with the Germans.
Fred enlists the help of Tommy and Jock, two British drivers who have been separated from their unit. While Fred and Ann delay the mayor and his men, Tommy and Jock load the machines on to their truck, and the four make their getaway.
They decide to head for La Rogette, where they will board a boat for England. Ann agrees to accompany them as far as Reauville, where she plans to join her sister Helen, a nurse. On the road, they come across a convoy of refugees, and offer a lift to a nun and a group of orphaned children. The convoy comes under German air attack, in which Fred receives a minor injury and the nun is killed.
Continuing on their way with the children in tow, they stop at a deserted farmhouse, where they manage to find food and fuel. Arriving at Reauville, they discover the town in flames, following a German bombing raid. The town Prefect advises Fred to take the children to a convent in Montreaux. Meanwhile, Ann discovers her sister has been killed in the bombing.
After dropping off the children in Montreaux, Tommy and Jock are reprimanded for deserting their unit by a British officer. They and Fred accompany the officer to his headquarters, but Jock realises that the officer is a German agent, and the three are forced to shoot their way out, picking up Ann before continuing on their journey.
Soon, they find themselves under attack from a German fighter plane. Despite managing to shoot the plane down, Jock is killed. On the way to La Rogette, the remaining trio come upon a unit of French troops preparing to blow up a bridge to stop the German advance. They get across just in time.
Discovering that the Germans have reached La Rogette before them, they head instead for St Paul, a small fishing port, dodging German troops and getting a flat tire in the process.
At St. Paul, they find a fishing boat preparing to leave for England, and ask the captain if he will take them and the machines. Refusing their offer of money, the captain decides to ask his passengers, desperate refugees fleeing the Germans, if they are willing to leave behind their luggage to make room for the machines. The passengers agree, and the boat departs with the machines on board.
