The narrative follows three inter-connected perspectives covering different but overlapping periods: on land covering one week, on the sea covering one day, and in the air covering one hour. The trilogy interweave to create a non-linear narrative.
An introductory text says that in 1940, after the invasion of France by Nazi Germany, thousands of Allied soldiers retreated to the seaside town of Dunkirk. As the Allied perimeter shrinks and German forces close in, the soldiers await evacuation in a seemingly hopeless situation.
On land, Tommy, a young British private, is one of several soldiers who come under fire from unseen German forces on the streets of Dunkirk. He is the only one to make it to the beach, where he finds British and allied troops waiting for evacuation. He meets Gibson, another young soldier, who appears to be burying a friend. After a German air attack, they happen upon a wounded man left for dead, and they rush his stretcher up to the front of the queue to a ship evacuating the wounded. They are denied passage themselves, and hide on the mole, hoping to sneak aboard the next vessel. However the ship is attacked as it departs; in the chaos they save another soldier, Alex, from being crushed by the sinking ship. They get on another departing boat that night, but this is sunk by a torpedo from a U-boat. Gibson saves Tommy and Alex and they are taken to shore by the shell-shocked soldier.