I was intrigued to hear of the lawsuit regarding Led Zeppelin's iconic Stairway to Heaven. The introduction to this song does sound similar to the Plaintiff's as others have identified. But I heard a musician discuss how common the particular musical phrase was; reviewing four other songs that had a similar opening passage, albeit in a different key or slightly different rhythm.
Composers have only eight notes to work with plus the notes' sharps or flats, depending on the key signature. So, for example, the Key of C would have the notes C D E F G A B plus some accidental sharps or flats. These notes can be arranged in infinite ways. Then there's the rhythm.
In a sense, writers are much better off with twenty-six letters arranged in a dictionary full of words. There's the old saw about a roomful of monkeys on typewriters. Given enough time, one of them will produce the complete works of Shakespeare. Or so it is postulated. Somerset Maugham admitted that "All the words I use in my stories can be found in the dictionary--it's just a matter of arranging them into the right sentences."
We are all the product of our past and the memories that go with that. A fragment from childhood, words or tune, might suddenly surface in our minds and declare itself original. We might search the recesses of our recollection in an ultimately futile endeavour and thereby reassure ourselves that there has been divine inspiration or the equivalent. We have been original and unique.
Of course, all artists are inspired by their predecessors or even their contemporaries. Shakespeare gleaned much from Greek and Roman mythology. It has been suggested that The Lion King is an anthropomorphized Hamlet. But, no lines of speech were directly lifted from the original. Neither Shakespeare nor the Greek and Roman storytellers receive any royalties today.
Romance novels have their tropes or commonly recurring literary and rhetorical devices, motifs or cliches. There are even acronyms: HEA for Happily Ever After or the modern equivalent: HFN - Happy for Now. A plot that has two people meet, encounter obstacles over which love ultimately prevails has surely been repeated, with variations, many times over.
What about the plot devices of mistaken identify or time travel or even amnesia? Familiar, even common, but is that plagiarism?
I have heard of unscrupulous people who download a free romance novel, go through the book making name, occupation and location changes as well as some other details. With a new cover and title the book is then uploaded as their original work. Royalties roll in and the original author is unlikely to discover the subterfuge. Even more so if the book was translated to a foreign language.
Almost everyone would agree that the foregoing is unlawful plagiarism.
(Since I drafted this post, the Courts have decided that there was no plagiarism in Stairway to Heaven.)