In 1917, Flight Lieutenant William Terrance Decker is returning from patrol over France during World War I. He passes through a cloud and lands his biplane on the airstrip at the Lafayette U.S. Air Base in Reims, France... and finds himself next to an Air Force military transport.
A jeep comes out and two guards order Decker to pull over. When he stops his biplane, they ask why he’s flying an antique and running it in front of an approaching aircraft. Decker identifies himself as a British and wonders why Americans are there. When Decker wonders how they became so advanced, Major Wilson takes him the base commander, General Harper. Wilson explains what happened and Harper demands to know who Decker is. Decker identifies himself as a member of the Royal Flying Corps, and Harper asks if he’s with an air show or making a film. The British lieutenant insists that he’s wearing a uniform, not a costume, and asks where he is. He admits that he thought he was landing at the 56th Squadron, RFC, and Wilson asks him what the year is. Decker identifies the year as 1917, and they explain that it’s March 5, 1959.
Decker realizes that they’re not joking, and insists that he isn’t either. When a jet fighter takes off, Decker hears it and looks up at the cloud that he passed through. He tells them that when he came through the cloud, the engine noise dropped off, and that something similar happened to a French pilot who disappeared without a trace. Decker mentions that he was flying with a partner, Mac, and Harper realizes that he’s talking about Air Vice Marshal Alexander Mackaye, who is en route to Lafayette. Decker says that’s impossible... because Mackaye is dead.
Harper and Wilson go over Decker’s belongings from the biplane, which seem authentic. They begin to wonder if it’s a hoax, since there’s no reason for anyone to go to such lengths to play a joke. Harper figures that Mackaye can provide some information on Decker when he arrives. Wilson begins to believe Decker’s story, and Harper says that he suspects that Decker may have been there to assassinate Mackaye. When Wilson questions his theory, Harper dismisses him.
Wilson goes to see Decker in the brig, and Decker demands to know why he’s being kept prisoner. He is reluctant to see Mackaye, and Wilson asks him why. Decker insists he’ll see Mackaye, and says that he knows Mackaye as “Old Leadbottom” when he was wounded by German fire in an embarrassing spot. When Wilson asks why Decker believes Mackaye is dead, the British pilot says that when he last saw Mackaye, he was surrounded by seven German planes. He was involved with three others and couldn’t get away, and doesn’t see any way that Mackaye could have gotten away. Wilson explains that Mackaye saved thousands of lives during the Blitz in World War II. Decker suddenly makes a break for the door but is quickly restrained. He insists that he has to leave because Mackaye will know him as a coward. Decker explains to Wilson that he’s always been a coward. When he went on patrols, he convinced Mackaye to split up and then lingered in the clouds, dreading the possibility of encountering an enemy plane. Wilson tries to reassure him, saying all pilots are afraid at one time or another and Mackaye will understand. Decker explains that he’s the one who let the Germans trap Mackaye because he abandoned his partner, and there’s no way he could have survived.
When Wilson suggests that Mackaye must have got help, Decker suggests that he’s the one who helped him. There was no one else within 50 miles, and he believes he was brought to 1959 to find out that Mackaye was alive so he could have a second chance. He insists that Wilson let him go so he can go back and save Mackaye 42 years ago. When Wilson refuses, Decker punches him and runs out to the airfield. When Wilson recovers, he sounds the alarm.
Decker finds his biplane and starts it. When a mechanic tries to stop him, Decker punches the man and gets in. Wilson arrives and holds him at gunpoint, demanding that he stop the plane. The British pilot refuses, insisting that he has to save Mackaye and all the people that he saved. He tells Wilson to shoot if he has to, but Wilson refuses and Decker takes off.
Wilson reports to Harper, who demands to know why he let Decker go. Before Wilson can answer, Mackaye arrives. Wilson asks him about Decker, and Mackaye explains that Decker saved his life in 1917. The Germans attacked him, and Mackaye noticed that Decker flew away at first, disappearing in a strange white cloud. But out of nowhere, he dived down and took out three of the German planes before they got him. Harper shows Mackaye the bag of Decker’s belongings that they kept, and Wilson refers to him as “Old Leadbottom.” They sit down and start to explain to Mackaye what happened.