Salesman Bob Wilson and his wife Julia board a flight and Bob ends up sitting next to the auxiliary exit window. Realizing her husband nervous, Julia offers to get them moved. Bob lights up a cigarette and admits that he isn't acting like a man who is cured. Julia assures him that he's fine now, and that she's missed him for the last six months when he had a breakdown on a similar flight. Bob jumps when they close the cabin door and admits he's suffering from confidence after his breakdown.
Once the plane takes off, Julia dozes off with a sleeping pill. She wakes up as they fly into a storm, but Bob assure her that everything is okay. Once Julia dozes off again, he looks out the window onto the wing of the plane… and sees something man-shaped standing there. As it crawls onto the engine cowling and then approaches the window, Bob signals for the stewardess. When he says there's a man out there, she doesn't believe him, and there's no sign of the creature. Bob apologizes and asks for a glass of water. Once she leaves, Julia asks what's wrong and Bob says he's just tired. He draws the window curtain, takes a sleeping pill, and tries to doze off.
A few minutes later, Bob can't resist opening the curtains… and he finds himself face-to-face with the creature, peering curiously at him. By the time he calls the stewardess over, the creature has moved away. Bob claims he called her over to ask about the storm. However, when she walks away, the creature returns and sits on one of the engines. It plays curiously with the blade and then notices Bob looking at it. It goes to the other engine and begins tearing at the parts.
Bob wakes up Julia and tries to explain, thinking it might be some kind of gremlin. He interprets her interest at her being patronizing, but asks her to tell the pilots simply to keep an eyes on the wings. Bob tells her that if they don't see anything, he'll commit himself. When she hesitates, he admits that he knows it's asking a lot of her. Julia goes to the cabin door and the stewardess intercepts her. While Julia asks her to get the flight engineer, Bob sees the creature again and calls Julia and the flight engineer over. He insists that there's a man on the wing and asks the flight engineer to look. There's nothing there by the time they look. The flight engineer humors him and tells him to keep calm to avoid frighten the passengers. Bob realizes what he's doing and tells him to stop, and says he'll let the plane crash rather than speak anything else.
As Julia goes with the flight engineer to get a sedative, Bob tries to determine if he's imagining things or not. He looks out and sees it toying with the engine again. When Julia returns, the creature floats away and Bob takes the sedative. Once Julia dozes off, Bob takes the pill out of his mouth and looks outside. The gremlin is toying with the engine again. Bob notices an air marshal behind him with a gun. He gets the sleeping man's gun and goes to the window.
As the creature continues sabotaging the engine, Bob fastens his seatbelt and then asks Julia to get him a glass of water. Once she's safely away, Bob opens the auxiliary hatch and is sucked out the opening. He manages to bring the gun to bear and shoots the creature repeatedly.
Later, the plane lands safely on the ground. Bob is placed in a strait jacket and Julia tries to reassure him. Bob tells her that he knows everything is okay, but he's the only one who knows. Julia and the crew are unaware for the moment that the gremlin's damage is present, and will soon be discovered, confirming Bob's claims.