Arthur Conan Doyle's estate is suing Netflix, among others, over its upcoming film "Enola Holmes" -- arguing that the show's depiction of Sherlock Holmes as kind, caring and respectful of women is a violation of the author's copyright. The film, based on a series of novels by Nancy Springer and set for release on Netflix in September, follows the legendary detective's younger sister, a character created by Springer. But the late author's estate has objected to the way Holmes is portrayed in the series, arguing that the sleuth was only ever kind and emotional in books that are still under the author's copyright. In earlier works, now in the public domain, his aloofness and lack of empathy are crucial aspects of his character and must be respected in any adaptation, the estate claims. Many later Sherlock Holmes titles are still protected under US copyright protection law. The estate has filed a case against Netflix, the US-based producers, Springer, her publisher Penguin Random House, and others in the US District Court for the District of New Mexico, just three months before the movie -- starring Henry Cavill as Sherlock and Millie Bobby Brown as the title character, Enola -- is set to premiere on the video platform. "While Sherlock Holmes is famous for his great powers of observation and logic, he is almost as famous for being aloof and unemotional," the filing argues, citing an extract from a Conan Doyle story in which his long-time friend and assistant Dr. John Watson describes Holmes as being "as deficient in human sympathy as he was pre-eminent in intelligence." "(T)o Holmes, Watson was utilitarian -- to be employed when useful, then set aside," the filing goes on. "Holmes did not treat Watson with warmth." While most of Conan Doyle's Sherlock Holmes stories are available in the public domain and can therefore be adapted by anyone, a series of later stories -- written after World War I, which had a profound impact on the author -- are still under copyright. The estate, which has gone after other alleged infringements over the years, argues that it was only in those later, copyrighted stories that the detective softened up -- and that by using those gentler character traits, the "Enola Holmes" books and film are therefore infringing copyright. CNN has contacted Netflix, Springer and Penguin Random House, the books' publisher, for comment. "Holmes needed to be human," the filing says, after describing the impact of the war on Conan Doyle. "He became capable of friendship. He could express emotion. He began to respect women." "[T]he Springer novels make extensive infringing use of Conan Doyle's transformation of Holmes from cold and critical to warm, respectful, and kind in his relationships," the estate claims. "Springer places Enola Holmes at the center of the novels and has (Sherlock) Holmes initially treat her coolly, then change to respond to her with warmth and kindness," it adds. It cites a passage from Springer's 2008 book "The Case of the Bizarre Bouquets," in which Holmes worries about Watson after he goes missing. "Nowhere in the public domain stories does Holmes express such emotion," the filing argues. The filing claims neither Springer nor her publisher nor the producers of the Netflix adaptation requested permission to use Conan Doyle's copyrighted stories. Conan Doyle died in 1930 after publishing dozens of stories about his legendary detective. Since his death, "Sherlock Holmes" has formed the basis of numerous films and TV series, including the popular BBC version, "Sherlock," which starred Benedict Cumberbatch in the title role. In its promotional material in April, Netflix said the new film "tells the story of Sherlock and Mycroft Holmes' rebellious teen sister Enola, a gifted super-sleuth in her own right who often outsmarts her brilliant siblings." It added that the film "puts a dynamic new female twist on the world's greatest detective and his brilliant family."