That was a misstatement by the court because plots are generally not considered copyrightable. The most famous statement on that point comes from Judge Learned Hand in Nichols v. Universal Pictures, 45 F. 2d 119 (2d Cir. 1930):

when the plagiarist does not take out a block in situ, but an abstract of the whole, decision is more troublesome. 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. The last may perhaps be no more than the most general statement of what the play is about, and at times might consist only of its title; but there is a point in this series of abstractions where they are no longer protected, since otherwise the playwright could prevent the use of his "ideas," to which, apart from their expression, his property is never extended. Nobody has ever been able to fix that boundary, and nobody ever can.

This abstractions test essentially says that there is a point somewhere between a highly detailed plot and a mere basic outline of a story where copyright protection ends.

So, for example, anyone is free to write a story about young magicians attending wizard school. That allows us to have things like JK Rowling's Harry Potter and Lev Grossman's The Magicians.

However, the closer the specific details of your plot tracks that of Harry Potter, the greater the risk of misappropriation because you're no longer taking the general idea but rather protected expressive elements. An example would be if your story revolves around an orphaned heroine who attends a space-based boarding school to learn how to be a Solar Knight after discovering that her parents were killed by the Evil Galactina, who was banished when she tried to kill our heroine and failed (leaving her with the mark of the starry eyes). At school, the heroine joins one of three groups of students, and plays space-hockey, and one teacher is threatening while another is nice, but in the end the nice one is Galactina's minion, etc.