September 1940. During the Battle of Britain, a squadron of airmen, joined by Station Commander Geoffrey Crisp, discusses the merits of the Spitfire. Crisp begins to reminisce.

1922. R.J. Mitchell dreams of designing a new kind of plane; one which applies avian principles and does away with struts and wires, with body, wings and tail all part of the essential structure. His wife Diana convinces him to suggest his revolutionary designs to his bosses but they ignore his ideas and assign him manual work in the assembly shop.

1923. The Schneider Trophy, an international airspeed competition, is won by America. Mitchell is now a designer but still working on the traditional designs his company insists on producing. An old school friend, Geoffrey Crisp, visits him. After serving in the air force during the First World War, Crisp has been unable to find permanent employment, and he offers his services to Mitchell as test pilot.

Mitchell's superiors offer him the job of designing the plane that will next be entered into the competition. He tells them that he will build them a plane to win, but only if given a free hand. They refuse, and he resigns. Commander Bride, managing director of the company, overrules the other directors and re-offers the commission to Mitchell on his own terms. He accepts, and immediately sets to work.

1925. Mitchell's new plane, piloted by Crisp, is entered into the competition. It flies superbly, but suddenly crashes into the sea. Crisp assures Mitchell that he had blacked out and lost control of the plane, and that no aspect of its design was to blame for the accident. Mitchell is given another chance in the 1927 competition. Crisp pilots the plane to victory, and sets a new speed record.

In 1929, Mitchell's plane wins a second time. At a celebration party, he meets Lady Houston, an eccentric socialite whose yacht is adorned with the slogans 'Down with the government' and 'Wake up England'. She tells him that she foresees a terrible danger, and is convinced that England must re-militarise itself to meet the threat. When he suggests that future wars may be won in the air she laughs at his ideas and implies that money spent on building a stronger air force would be wasted.

The rules of the Schneider Trophy competition state that if a country wins three years in succession the prize becomes theirs permanently. But the British team lack the money to even enter the competition that may secure them this permanent victory. They approach the government, but their proposals are turned down as an inappropriate use of public money. All looks black until Lady Houston, who has had second thoughts about Britain's air defences, supplies the money. Mitchell wins yet again, setting another new speed record, and is made C.B.E.

A few years later, Crisp, Mitchell and Diana holiday in Germany. They discover that Germany is defying the prohibitions on the manufacture of weaponry set out in the Treaty of Versailles and is beginning to prepare for a future war. They return to England to attempt to convince the government of the urgent need to rebuild Britain's military defences. With limited funds and little official support, Mitchell designs the plane that will become known as the Spitfire. His health, however, suffers from the pressure, and his doctor tells him that he must cease all work immediately; if he continues to exert himself at the present rate he has only eight months to live. Returning to his office, he learns that the government has finally accepted his designs, but the plane must be ready in twelve months. Mitchell replies that he will have it ready in eight months, and goes back to work.

The plane is finally ready for trial, but the effort has left Mitchell an invalid. On the day he learns that official permission has been given to mass-produce the planes, Mitchell falls asleep under a blanket in his garden, and Diana begins to weep.

1940. Crisp confirms that Mitchell died. The squadron is called back to the battle. After a successful mission Crisp thanks Mitchell, and the planes fly home.