May 1940, and Corporal Tubby Binns is with the Wiltshire 13th Brigade of the British expeditionary force in France. When the German army invades Belgium, his company are ordered to leave their prepared positions in an attempt to bolster Belgian defences, but they are rapidly pushed back into France.
In London, journalists such as Charles Foreman question the military tactics and the fact that Ministry of Information censorship prevents these huge troop movements from being reported. Foreman is scathing of those failing to take the war seriously and those he considers to be profiting from the war, such as garage owner John Holden, who is manufacturing buckles for the army. He also encounters despondency from a French correspondent who fears that military miscalculation is losing the Allies the campaign.
After a mission to bomb a bridge, Corporal Binns' unit finds that its division has pulled out, and when his lieutenant is killed by enemy aircraft, he finds himself reluctantly in command of the isolated group of five men. After witnessing enemy aircraft attack a column of refugees, they manage to locate a Royal Artillery battery. Their relief is short-lived however when shelling results in heavy casualties, including one of Binns' men. The commander of the battery realises the impossibility of successfully defending their position, and dispatches Binns' unit along with a few other men to join up with their company. As soon as Binns and his men have left, the battery position is destroyed by Stuka bombing.
The British military hierarchy withdraw divisions attacking southwards in order to protect the escape route for the troops heading north. After the shelling of Calais, Dunkirk is seen as the only viable option for evacuation. Under Operation Dynamo, all available ships and boats are requisitioned to reinforce the Navy. Foreman is ordered to take his boat Vanity to Sheerness, but Holden initially refuses to accompany his boat Heron, citing the responsibilities of being a new father. His conscience soon gets the better of him and he relents. At Sheerness, Foreman suggests that he and the others should sail their own boats to Dunkirk. After his underage employee Frankie also volunteers, Holden agrees to accompany his vessel.
Binns and his men take refuge in an abandoned farm but an exchange of gunfire with some German soldiers the following morning leaves one of their number badly wounded, forcing Binns to leave him behind. This unpopular decision prompts some of the men to question Binns' ability, but he manages to assert his authority, leads them through enemy lines and gets the unit onto a British army truck. After six days separated from their division, the men make it to the beach at Dunkirk, where the massed troops are awaiting evacuation. The Navy's initial decision to withdraw vessels as a protective measure is reversed, and further boats are dispatched.
Binns and his men make it onto a boat, but after bombardment by aircraft, they end up back on the beach. Foreman's boat is also hit and he is rescued by Holden. At a Sunday morning prayer service on the beach, Stuka planes attack the troops and Foreman is killed. One of Binns' men is able to repair the Heron's damaged engine, and Holden ferries the men off to a Navy vessel. Binns, Holden and Frankie make it back to England, along with the other third of a million men to be rescued from the beaches at Dunkirk.
