In 1940, during the fall of France, hundreds of thousands of Allied soldiers have retreated to Dunkirk. Tommy, a young British private, is the sole survivor of a German ambush. At the beach, he finds thousands of troops awaiting evacuation and meets Gibson, who is burying a body. After a German dive-bomber attack, they find a wounded man. They rush his stretcher onto a hospital ship, but are denied passage themselves. The ship is sunk by dive-bombers; Tommy saves Alex, another soldier. They leave at night on a destroyer, which is sunk by a torpedo from a U-boat. Gibson saves Tommy and Alex, and they are brought ashore by a rowing boat.

The British navy requisitions civilian vessels that can get close to the beach. In Weymouth, Mr. Dawson and his son Peter set out on his boat Moonstone rather than let the navy take it. Impulsively, their teenage friend George joins them. At sea, they rescue a shell-shocked officer from a wrecked ship. When he realises that Dawson is sailing for Dunkirk, the officer demands that they turn back, and tries to wrest control of the boat; in the struggle, George suffers a head injury that renders him blind.

Three Spitfires head towards France. After their leader is shot down, pilot Farrier assumes command with a shattered fuel gauge. They save a minesweeper from a German bomber, but the other Spitfire is hit and ditches. Its pilot, Collins, is rescued by the Moonstone.

Tommy, Alex and Gibson join a group of Scottish soldiers and hide inside a trawler until the tide rises. Its owner, a Dutch mariner, returns. German troops shoot at the boat for target practice; when the tide rises, water enters through the bullet holes. Alex, hoping to lighten the boat's load, accuses Gibson, who has been silent, of being a spy and demands that he leave. Gibson reveals he is French; he had stolen the identity of the soldier he buried, hoping to evacuate with the British. Alex, Tommy and the Scottish soldiers abandon the fishing boat when it begins to sink. Gibson is entangled in a chain and drowns. Alex and Tommy swim towards a nearby destroyer, but it is sunk by a bomber. The Moonstone manoeuvres to take on troops, including Alex and Tommy. Peter realises that George is dead; when asked by the shell-shocked officer, he lies that George will be fine. Farrier shoots down the bomber, which crashes and ignites the oil slick from the sinking destroyer. Peter reveals to Collins that his elder brother was a Hurricane pilot, killed early in the war.

Farrier reaches Dunkirk, his fuel exhausted. Gliding over the beach, he shoots down a dive-bomber to cheers from the troops below. He cranks his landing gear down and lands beyond the Allied perimeter. He sets fire to his plane and is taken prisoner by German soldiers.

At the beach, Commander Bolton watches the last British soldiers leave. He confirms that 300,000 have been evacuated, ten times more than Winston Churchill had hoped for. He remains to oversee the evacuation of the French rearguard. Alex and Tommy cross the English Channel and board a train in Weymouth. Dawson is congratulated for having saved so many men. The shell-shocked officer sees George's body being carried away. Peter visits the local newspaper with a photograph of George; a front-page article later commends George as a hero. Alex expects public scorn as the train approaches Woking, but they receive a hero's welcome. Tommy reads out Churchill's address to the nation from a newspaper.

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