The Gremlin is a major antagonist in The Twilight Zone, having appeared in Nightmare At 20,000ft as well as returning in Twilight Zone: The Movie - it is considered by many to be one of the most famous monsters to come from the show alongside Anthony Fremont.
The Original Tale
Bob Wilson (William Shatner) is a salesman on an airplane for the first time since his nervous breakdown six months earlier. He spots a gremlin on the wing of the plane. Every time someone else looks out the window, the gremlin leaps out of view, so nobody believes Bob's seemingly outlandish claim. Bob realizes that his wife is starting to think he needs to go back to the sanitarium, but also, if nothing is done about the gremlin, it will damage the plane and cause it to crash. Bob steals a sleeping policeman's revolver, and opens the window marked "Auxiliary Exit" to shoot the gremlin, causing it to fall off the wing to its death, despite the fact that he is nearly blown out of the plane himself. Once the plane has landed, although he is whisked away in a straitjacket, a final shot reveals evidence of his claims: the unusual damage to the plane's engine nacelle—yet to be discovered by mechanics.
This episode was remade into a segment of the 1983 movie version of the series, with John Lithgow portraying the main character, who has been renamed John Valentine. The story is somewhat shortened, but the plot in general is the same, although with some differences. In this version, Valentine travels alone, and his fear of flight seems to be more emphasized, as the segment begins with an almost hysterical Valentine hiding in the bathroom. When he eventually spots the gremlin, he reacts more strongly than the original incarnation of the character. He yells at the flight crew and his fellow passengers on several occasions. At the end of the segment, in a scene not shown in the original 1963 TV episode, the mechanics discover the severe damage done to the plane.
The original gremlin was an ape-like creature which seemed to be driven by curiosity rather than a will to cause damage. In the movie, the gremlin more resembles an alien, with slimy beige skin and a frightful grin. It seems more intelligent and menacing, beginning to dismantle one of the jet's engines, rather than curiously roaming about as the original gremlin did. It taunts Valentine several times, holding up a piece of wing and demonstratively tossing it inside the engine to damage it. When Valentine tries to shoot the gremlin, it runs over the wing to Valentine, grabs his hand holding the gun, and promptly bites the gun in half. At that moment the lights of the landing field appear below. The gremlin grabs Valentine's face, seemingly about to kill him, but stops and then waves its finger in a dismissive "tut-tut-tut" manner. It then leaps away, off into somewhere else. The original gremlin never made physical contact with Wilson.
The epilogue features Valentine being driven to the sanitarium by the passenger from the prologue (played by Dan Aykroyd), who is actually a demonic creature who killed his driving companion (Albert Brooks) at the beginning of the movie after asking, "Want to see something really scary?". He poses the same question to Valentine just as the film ends, which then worries him.