In 1940, during the Battle of France, Allied soldiers retreat to Dunkirk encircled by the enemy. Tommy flees through the perimeter held by French troops to the beach, where thousands await evacuation, and helps Gibson to bury a body. After Luftwaffe dive-bombers attack, they attempt to board a hospital ship at the single, vulnerable mole available for embarking on deep-draft ships, by rushing a wounded man on a stretcher but are ordered off. They overhear Commander Bolton, Colonel Winnant and a Rear Admiral discuss the best way to get their army evacuated. The ship is sunk by dive bombers; Tommy saves a Highlanders regiment soldier, Alex. The three board a destroyer, but it is hit by a torpedo before it can depart; Gibson saves Tommy and Alex as the ship sinks, and they return to the beach.
The Royal Navy requisitions civilian vessels in England to get to Dunkirk. In Weymouth, civilian sailor Dawson, with his son Peter, set out in his boat Moonstone, rather than let the Navy commandeer her. Their teenage hand George joins them on impulse. In the English Channel, they save a shivering shell-shocked soldier from a ship destroyed by a U-boat. Realising that Dawson is going for Dunkirk, the soldier panics and Peter locks him up. The soldier escapes, urging they turn back and tries to wrest control of the boat; in the scuffle, he elbows George who suffers a head injury that blinds him; as the soldier dwells on his actions, George reveals to Peter he came hoping to do something noteworthy. Three Royal Air Force Spitfires fly towards Dunkirk, to provide cover for the evacuation, limited to one hour of operation by their fuel supply. They engage in a dogfight with an enemy fighter. One of the pilots, Farrier, has his fuel gauge smashed by another fighter. He and the second Spitfire pilot, Collins, determine that their leader has gone down and fly on. The crew of the Moonstone witness the two RAF pilots protect a minesweeper from a bomber: Collins’s Spitfire is hit by a fighter and he ditches. Although trapped in his canopy as the plane sinks, Collins is saved by Peter.
Tommy, Alex and Gibson and Highlanders soldiers hide in a grounded trawler in the intertidal zone outside the perimeter, waiting for the rising tide. After its Dutch sailor returns, Germans start shooting at the boat for target practice, and water enters through the bullet holes. Alex, attempting to lighten the boat, accuses Gibson, who has been silent, of being a German spy. Gibson reveals he is French; he took the identity of the British soldier he buried. The group abandons the sinking boat, but Gibson is entangled in a chain and drowns. Farrier chooses to continue aiding the evacuation, despite realising that he will never make it home. The destroyer is bombed and sinks, as Moonstone manoeuvres to save men in the water, including Alex, as the shivering soldier starts helping. Peter finds George is dead; asked by the shivering soldier, he says George will be fine. Farrier shoots the bomber down; its crash ignites oil on the water, but Peter saves Tommy. Farrier reaches Dunkirk just as his fuel runs out. Gliding, he shoots down a dive-bomber approaching the mole, and is cheered on by the troops. Farrier lands his Spitfire on the beach beyond the perimeter, burns it and calmly awaits capture. Dawson has the boat evade aerial attack, using a technique taught by his deceased elder son, a pilot lost at the start of the war.
With 300,000 men successfully evacuated, Commander Bolton stays to oversee the French evacuation. In Weymouth, the shivering soldier sees George's body and exchanges a glance with Dawson, as he and Collins depart. Tommy and Alex board a train with other soldiers and are heralded by the public at Woking. Tommy reads Churchill's address, encouraging Britain to fight on. Peter arranges for the media to eulogise George.
