US Army sergeant John Patterson arrives with his pal Sgt Greenbrugh at their billet on the 2nd floor of 20 Grosvenor Square, the London residence of the Duke of Exmoor. Exploring the house, Patterson finds housekeeper Mrs Wilson cool towards the new guests. Patterson summons a taxi. British army Major David Bruce offers to share, and in a gesture of friendship invites him to his military club.

At a Hampshire RAF base, the Duke's granddaughter, Lady Patricia Fairfax, is a driver. She greets Bruce very formally, but once out of sight they embrace.

Patterson makes friends with the Duke, who invites the Americans to visit Exmoor Castle, his Devon seat. On their arrival, Bruce is playing tennis with Patricia (who is his fianceé), and they later gather for afternoon tea. Patricia mentions the local by-election, in which Bruce is standing as the Tory candidate. After the visit, Patricia and Patterson return to London by train. With several hours to spare, he takes her to 'Rainbow Corner', in Piccadilly Circus, where they dance. Patricia makes the last train to Hampshire.

Patricia and Patterson go riding on Exmoor. While she is away, Bruce addresses a political meeting in the village square. On the eve of polling, the election is discussed in the local inn and at a formal champagne dinner dance at Exmoor Castle, where Patricia and Patterson declare their mutual love on the terrace. Bruce soon realises that he has lost Patricia's affections.

Just before the election result is announced, the Duke (who is also Mayor) learns that his castle is to be requisitioned by the US army. He then has to announce a majority of 492 for Labour, ending 300 years of local tradition.

Back in their billet, Patterson learns that he is to go on operations. The Americans take Mrs Wilson to 'Rainbow Corner' to celebrate her birthday, and she warms to them. Indeed, she likes Patterson so much that she later offers him some of her savings

As the Duke prepares for the US army moving in to his castle, Patterson bids farewell; the Duke gives him his airman son's watch from WWI.

Patricia receives her officer's commission, and at the Savoy she and Bruce have a farewell dinner. The Duke arrives with Patterson, and leaves the two love rivals at the table. As Lady Patricia dances with Bruce, Patterson quietly leaves. Later in the evening, in a noble gesture of self-sacrifice, Bruce places a phone call to Patterson, then asks Lady Patricia to take the call. She and Patterson meet up and talk for a final time before his mission.

Returning to England after the mission, Patterson takes over the controls from the injured co-pilot. As they are South West of Exmoor flying on one engine, they pull the plane clear of Exmoor Castle and the village before it crashes. All the airmen are killed. Air control reports a list of the casualties to Patricia, who is on control room duty, and she is greatly distressed.

When Major Bruce's airborne mission is delayed for 24 hours, he arranges to meet Patricia for a farewell drink, and they talk of the tragic plane crash. News of Patterson's death is also received by his mother in Arizona.

In the Exmoor village church, the vicar gives a eulogy for the brave men who gave their lives as the names of the dead flyers are read out.

Bruce flies on his paratroop mission with a Dakota pilot from Buffalo, New York.