Summer 1941. A Wellington bomber flies low over the North Sea without any crew to steer it home, as if it were the Mary Celeste. Suddenly, it hits an electric pylon over Eastern England and explodes.
Fifteen hours earlier, the six crewmembers of B for Bertie prepare to take off on what they hope will be a routine bombing raid over the city of Stuttgart. The crew are young and fresh, except for veteran rear gunner Sir George Corbett, who complains that the cramped conditions are not to his liking. The droning engine propels the heavy bomber slowly onward while the crew fix its flight coordinates. For a time, all they see is the expanse of the water as they cross the North Sea towards the Netherlands, where they hear the sound of anti-aircraft guns.
The men remain cheerful and full of humour, despite the danger of their mission. B for Bertie flies through heavy gunfire and loud explosions that light the night sky. Enemy fire damages one of the engines, and the crew are forced to parachute to safety over occupied Holland. B for Bertie flies on totally unmanned and, regaining power of its second engine, drives for home.
A group of children, out playing with their dog, is surprised to find the Royal Air Force up a tree. The local schoolteacher Els Meertens helps the men go into hiding, offering them cover within the small Catholic community. Risking their own lives, the villagers give the men clothes and clogs to disguise themselves. A network of resistance takes them by road to the coast, where they meet Jo de Vries. De Vries is a fearless woman, who tricks the Nazis into believing that she is on their side while she plans their escape.
De Fries navigates them through a series of waterways until they reach the main estuary and the open sea. However, they are spotted by a sentry on top of the lock. Just as they are about to reach the mainland, the sentry takes aim at the rowing boat, shooting George in the back. Floating in the North Sea, they are eventually picked up by a British patrol boat and towed back to England.
Back at the airforce base, George makes a full recovery and reports for duty on what will, he hopes, be a routine trip.
