Thirty-seven-year-old salesman Bob Wilson had a breakdown six months ago on an airplane, and has spent the time since in a sanitarium. His wife Julia has come to bring him home, and they're returning by commercial jet. They end up sitting with Bob next to the auxiliary exit on the wing, and Julia offers to move. Bob refuses and lights up a cigarette, and then admits that he isn't acting alike a cured man.
After the plane takes off, Julia takes a sleeping pill and dozes off. As they hit a storm, Bob looks out the window… and sees a man-like creature highlighted by the lightning. Bob calls the stewardess over but when she looks outside, there's nothing there. Bob simply asks for a glass of water as Julia wakes up. He tells her that he's tired, takes a sleeping pill, and tries to go to sleep. However, he can't resist looking out the window again… and finds the monkey-like creature staring in at him. Horrified and sweating, Bob calls the stewardess over but the creature has drifted off the wing and out of sight. After she goes, it comes back and starts messing with the engines, prying open a panel.
Bob wakes up Julia and tries to explain. He wonders if it's one of the "gremlins" that pilots described during the war. However, he soon realizes that she doesn't believe him and is just humoring him. Bob admits that his story does sound insane, but asks her to at least ask the flight engineer to keep an eye on the engines. If he's right, then all well and good. If he's wrong, Bob agrees to have himself committed again. Julia has the stewardess get the flight engineer, while Bob sees the creature outside again. When the flight engineer comes over, it's gone again. The flight engineer claims to have seen it as well but Bob quickly realizes that the man is humoring him and tells him to go.
Not sure if he is insane or not, Bob tries desperately to figure out if the gremlin is there or not. He sees it prying up the engine panel again, but it floats away when Julia returns. Bob takes the sedative she got from the flight engineer and seemingly swallows it. However, once Julia has gone back to sleep, Bob spits the pill out. He sees an air marshal carrying a gun. Passing by the sleeping man, Bob drops his cigarettes and takes the gun when he leans over to pick them up.
Bob then returns to his seat, fastens his seat belt, and asks Julia to get him a drink of water. Once she's left, Bob opens the hatch and shoots at the gremlin until it collapses.
Later, the plane lands and Bob is taken away in a strait jacket. Julia tries to assure him that everything is all right. Confident that what he saw was real and not a relapse, Bob says that it is, but he's the only one who knows it… yet. Behind him, the damage on the engine will soon be discovered, proving his sanity to everyone.