The Standard of Review. As for your question, the standard of review is best described by Judge Learned Hand in Nichols v. Universal Pictures in which the author of the popular play, Abie's Irish Rose, sued the producers of a movie, The Cohens and the Kellys. Both plots involved children of Irish and Jewish families who marry secretly because their parents are prejudiced. At the end of each work there is a reconciliation of the families, based upon the presence of a grandchild. Beyond that, the works had little in common except for ethnic clichés.
The Abstractions Test. Judge Hand established a standard to separate the idea from the expression. He used the term "abstraction," which is a process of removing or separating something. He stated: "Upon any work, and especially upon a play, a great number of patterns of increasing generality will fit equally well, as more and more of the incident is left out." In other words, every narrative work is built around an underlying idea, in this case the basic plot summary. The idea may be similar to other plots (BTW, many people believe there are only between seven and 30 basic plots), but the author's embellishments -- the series of details and incidents that separate the idea from similar plots -- trigger copyright protection. Copyright only extends to each author's unique expression, not the underlying idea.
Still in Use. Judge Hand's abstractions test is still applied for plots (and was even modified for application in software infringements). In Litchfield v. Spielberg a writer sued the makers of the movie E.T. -- The Extra Terrestrial. The writer claimed that the film infringed her musical play, Lokey From Maldemar, a social satire designed to "illustrate the disunity of man, divided by egotism." The district court applied the abstractions test and determined that the only similarity in both works was the basic plot line -- aliens with powers of levitation are stranded on earth, pursued by authoritarian characters and finally bid their earthly friends farewell. Again, these similarities (sometimes known as nonliteral similarities) are ideas and are generally not protectable.
Our Takeaway Points ... Nobody but a judge or arbitrator can safely tell you whether you've taken the "expression" of a plot or the "idea" of a plot, but you will likely run into problem if you have a high profile work and you're slavishly copying the plot (and/or characters) of a well known work. For example, that's what happened in the copyright battle between Star Wars and Battlestar Galactica (Chaim Green R.I.P.).