The movie ‘‘The Curious Case of Benjamin Button’’ is loosely based on a short story by F. Scott Fitzgerald. It feels like it’s based on a collection of short stories. As director David Fincher and screenwriter Eric Roth take the viewer through the life of a man who ages backwards, the two-hour-and-40-minute film has an episodic structure. And there are magical moments scattered throughout, from heart-wrenching stories to engaging interludes to scenes that demonstrates Fincher’s technical vision and his ability to execute it on screen. But ‘‘Button’’ is a movie that feels like less than the sum of its parts. While any 20 minutes of the movie is enchanting, it never builds to a wholly satisfying conclusion. The closest thing offered to an explanation for this curious case is that Benjamin (Brad Pitt) is born around the time a new clock tower is unveiled. The clock runs backwards, according to its designer (Elias Koteas), in order to bring back the boys lost in World War I.