Although the story and the film share many commonalities, the differences lie in making a short story long enough to suit a feature-length film which do not change the basic elements of the story's plot.
The first change is that Neddy is referred to as Ned. Shirley Adams, Neddy's previous mistress, also undergoes a name change. She is no longer simply described as being beautiful but is changed into an actress named Shirley Abbott. Shirley is given an opportunity to share her experience with being Ned's mistress, while the story version of Shirley never speaks. Several minor characters in the film are completely absent in the short story: Julie, the Merrill daughters' old babysitter; Kevin, a boy who Ned helps conquer his fears of swimming; and Joan, an eager young woman whose friends dissuade her from joining Ned on his adventure.
The film also hints at Ned's dark past much earlier in the narrative than the story by allowing the Westerhazys to appear concerned as Ned leaves their pool. In contrast, Neddy is well into his adventure before the text hints at his forgotten past.
Alcohol appears repeatedly in both the book and the film as well as Ned's inability to remember painful events from the past. The final scene in both the story and the film are the same: Ned returns to his deserted home and is unable to enter.