The film opens during the Battle of France in May 1940, with RAF pilots evacuating a small airfield in advance of the German Blitzkrieg. The pilots, along with British and French military, leave just as German aircraft arrive and execute a heavy strafing attack. RAF Air Chief Marshal Hugh Dowding (Laurence Olivier), realizing that an imminent invasion of Great Britain will require every available aircraft and airman to counter it, stops additional aircraft being deployed to France so that they are available to defend Britain.The camera looks over of the deserted beaches of Dunkirk while the BBC relays British Prime Minister Winston Churchill's declaration that "what General Weygand called the battle of France is over, the Battle of Britain is about to begin". Albert Kesselring arrives to inspect a large German airfield in captured France. Hundreds of Heinkel bomber aircraft are stationed under his command.Luftwaffe commanders are stunned when the Fuhrer informs them that the British are not their "natural enemy" and delays their attack while attempting a diplomatic settlement. In neutral Switzerland, the German ambassador (Curt Jürgens) officially proposes new peace terms to his British counterpart (Ralph Richardson), stating that continuing to fight the "masters" of Europe is hopeless. The Briton replies that his country will fight on. In England, commanders celebrate their good fortune, using the delay to build up their strength and continually train their pilots and ground controllers.The wait finally ends when Luftwaffe pilots receive orders to move to the front, where troops are preparing for a sea-borne invasion. The campaign begins with the Luftwaffe launching an early morning assault on "Eagle Day". The plan is to destroy the RAF on the ground before they have time to launch their Spitfire and Hurricane fighters. The Luftwaffe is aware of the British radar installations, and plan to attack them with their Stuka dive bombers. The fighter pilots question this, wondering if it is better if the British are warned by radar and come up to fight, where they can be shot down.Eagle Day proves highly successful, knocking two radar stations out of action and heavily bombing a number of the British airfields. British losses are relatively light, but the RAF is in the process of building its strength, and many of the pilots who lack combat experience are shot down. The airfields are heavily damaged, with many aircraft destroyed on the ground, and their ability to repair and rearm seriously affected.Adding to the RAF's problems is a battle between the commanding officers of 11 Group, Keith Park (Trevor Howard), and 12 Group, Trafford Leigh-Mallory (Patrick Wymark). 12 Group is tasked with protecting 11 Group's airfields while 11 Group meets the enemy, but in raid after raid 12 Group aircraft are nowhere to be seen. Called to meet Dowding, Leigh-Mallory explains that the "Big Wing" tactic takes time for form up, while Park complains that the tactic simply isn't working. Dowding ends the debate by pointing out it makes little difference; the problem is not the tactics but a critical shortage of pilots. Dowding wearily remarks, "We're fighting for survival, and losing".The turning point occurs when a squadron of German bombers becomes lost in bad weather at night and drops bombs on London. In retaliation, the RAF launches an attack on Berlin. Though the damage is negligible, Adolf Hitler is enraged, and publicly orders London to be razed. Hermann Goring (Hein Riess) arrives in France to personally lead the attack, confident that the end of the battle nears. Their first attack skirts the RAF, who are still defending their airfields to the south, and they bomb unopposed. Nighttime attacks follow, and London burns.Meanwhile, to supplement Commonwealth forces, the RAF has been forming units of foreign pilots who have escaped German-occupied countries; the main difficulty is their lack of English-language skills. While on a training flight, a Free Polish squadron accidentally runs into a flight of German bombers without escort. Ignoring the commands of their British training officer, they peel off one by one and shoot down several of the bombers. Park rewards them by elevating them to operational status, leading Dowding to do the same for the Canadian and Czech squadrons as well.While discussing the day's events, Park and Dowding examine the German switch to London. Given a respite, Park notes that he will be able repair his airfields and bring his squadrons back to full strength. Dowding adds that 12 Group units north of London are now all within range, while enemy fighters are at the extreme edge of their own range. He concludes that "turning on London could be the German's biggest blunder."The next German daytime raid is met by a massive response; watching his formations build up in 11 Group's operations room, Wing Commander Willoughby (Robert Flemyng) wryly states "this should give them something to think about." RAF fighters are arranged into large groups and attack with all force, overwhelming the Germans and breaking up their raid. Goring is incensed, ordering his fighters remain with the bombers, an order the pilots hate. Losses continue to mount on both sides.The climactic air battle of 15 September 1940 arrives, with Winston Churchill in attendance at 11 Group's operations room. In the underground bunker, British ground control personnel order every squadron into the air to meet the massive attack. Intense combat over the London sky follows, with both sides taking heavy losses. The outcome is so confused that Dowding refuses to pass comment on the events.The next day the RAF anxiously await a raid that never comes. Two German anti-aircraft gunners, who had earlier protected a French port teeming with Kriegsmarine vessels and landing barges, now observe a deserted harbor basin. Goring leaves the front, accusing his commanders of betrayal. As the campaign draws to a close, the words of Winston Churchill appear onscreen: "Never in the field of human conflict was so much owed by so many to so few.