A story is usually both an idea and a form of expression. It is typically written in the form of a draft or a script or an outline, and those are all forms of expression. a movie is also a tangible form of expression and will be protected by copyright. If all that is copied is the general idea, such as "a cute alien arrives and teams up with a young boy" that will not violate the copyright on ET. But if detailed, scene-by-scene plot, let alone dialog, is copied, that might well be a copyright infringement. Since this particular case was settled, there is no way to know what the court would have ruled on whether protected elements were copied, but there have been cases where a movie was based on a book or another movie and held to infringe.
I have read, but cannot confirm, that the settlement was for 15% of the receipts of A Fistful of Dollars. If this is true, then the defendants must have thought that the plaintiffs had a significant chance of winning a judgement. But that still does not say how a court would have ruled.