THE SAME basic plotlines form the basis for thousands of stories. A joke has it that there are only two plots: a stranger arrives, or a man goes on a journey. Ursula Le Guin’s “A Wizard of Earthsea” and J. K. Rowling’s “Harry Potter” series both feature a young orphan boy who discovers he has magical powers, attends a wizarding school and defeats an evil adversary, but no one would argue that they tell the same story. (Much the same could be said of Luke Skywalker.) Several recent lawsuits regarding alleged copyright infringements raise an important question. When it comes to an overlap of theme, plot or character, how close is too close?

“The Shape of Water” (pictured), the best-picture winner at this year’s Oscars, is the subject of a lawsuit brought by the estate of Paul Zindel, a Pulitzer prize-winning American playwright. The estate claims that Guillermo del Toro’s film “brazenly copies the story, elements, characters and themes” of Zindel’s play “Let Me Hear You Whisper” (1969). Both stories are set in a government-controlled laboratory during the cold war. Both feature a cleaner who develops a relationship with a captive sea creature, dancing in front of its tank with a mop to a love song and devising a plan to smuggle it to safety in a laundry cart after she learns of plans to kill and dissect it.

There are notable differences, and Mr Del Toro denies that the production team had heard of the play prior to the lawsuit. Yet even if there were proof that they had, Zindel’s heirs might find that the case isn’t clear-cut. Mark Litwak, an entertainment attorney who specialises in copyright, trademark and intellectual-property law and often serves as an expert witness in motion-industry disputes, says that copyright lawsuits based on plot similarities are notoriously difficult to win: it is not legally possible to own an idea. “Concepts, themes, subject matter—those are inherently not copyrightable,” he says.

American copyright law protects “the expression of an author”, a concept that is hard to define. It primarily prevents verbatim reproductions of an author’s words, but it can also encompass finely drawn characters and detailed plot points as well as narrative progression. Yet there is no clear ruling as to how many elements of an author’s work need to be mirrored before an accusation of copyright violation holds weight.

The onus is on the plaintiff to prove that infringement has occurred and that what has been copied is “more than an idea”, as Mr Litwak puts it. Unlike patent law, the first person to publish a creative concept does not own it. “If two people come up with the same story independently, without borrowing from the other one, they both get the copyright in their work,” he explains.

That doesn’t mean that ideas are without value—or that they can’t be pitched and sold. Another lawsuit is currently in progress over “Stranger Things” (pictured below), a hit drama from Netflix, co-written by Matt and Ross Duffer. Charlie Kessler, who also writes for the streaming site, claims he pitched the idea for a feature to the brothers when he met them at a film festival in 2014. He states that his ideas—explored in a short film he produced in 2012—were the inspiration for the hit series, which launched in 2016. He is not accusing the Duffer brothers of copyright infringement but of “implied breach of contract”. The Duffer brothers deny that the conversation ever took place.

What makes conceptual plagiarism particularly difficult to define and prove is the likelihood of authors developing similar ideas independently. “Brilliant minds think alike—and also mediocre minds think alike,” says Mr Litwak. “When the New York Times publishes a story about something there’s probably a dozen writers in Hollywood who read it and use it as a jumping-off point to write a plot, and they’re not copying each other but they come up with stuff that’s somewhat similar.” This is particularly true of genre fiction, which adheres to certain formulas and tropes. Both “Stranger Things” and Mr Kessler’s short film share similarities with a book called “The Montauk Project: Experiments in Time” (1992), about secret experiments conducted at a government facility on Long Island.

John Krasinski’s post-apocalyptic horror film “A Quiet Place”, which was released last month, shares several plot similarities with “The Silence”, a novel from 2015 by Tim Lebbon. Both stories are set in a future where monsters that hunt by sound are decimating humanity, and feature a family with a deaf daughter seeking shelter in the countryside. Although there are key differences—particularly with regards to the nature of the creatures—“A Quiet Place” left fans of Mr Lebbon confused. Several enquired whether it was a long-awaited adaptation of his novel. Mr Lebbon responded that the film’s similarities were “a little troubling” but wrote that he was confident that the adaption of his novel—which is due out later this year—“will stand on its own”. Inevitably “The Silence”, simply for arriving second, will be perceived to have tiptoed in the footsteps of “A Quiet Place”.

Perhaps a clearer legal framework that formally protects detailed plot development, as well as characters, would help writers safeguard their original ideas. But even then, assessing whether or not infringement has occurred is likely to be a subjective matter. Mr Litwak cites two similar cases in which authors admitted to taking an idea from an existing work, based on historical characters, but denied copyright infringement. In one, the judge ruled that the idea was not subject to copyright and the plaintiff lost. In the other, the plaintiff won on the basis that several overlapping plot elements constituted infringement.

While it is frustrating for authors to put time and effort into creating original stories only to see them mirrored elsewhere, there is a reason that ideas remain free. The plot may provide the backbone of a story, but it is the details that can transform a hackneyed concept into something innovative. Readers, for their part, often enjoy spotting connections and similarities. The coming of a decades-long winter, accompanied by invading northern tribes and fearsome snow monsters in Le Guin’s 1966 novel “Planet of Exile” are echoed to great effect in a different fantasy setting in “Game of Thrones”. “A Wizard of Earthsea” is wonderful, but it doesn’t mean that readers have no room in their hearts for “Harry Potter”.