Emma Cline’s 2016 novel “The Girls” was, by any measure, a triumphant debut. The novel, which takes place in California in the 1960s and centers on a teenage girl who falls into a Charles Manson-like cult, sold to Random House for seven figures in a three-book deal, and spent three months on the New York Times best-seller list. Ms. Cline, who was 27 when the novel came out, was celebrated as a major new talent.

But for the last two years, her success has been overshadowed, in private, by legal threats levied against her by a former boyfriend, Chaz Reetz-Laiolo.

This week, that long simmering dispute became glaringly public, when Ms. Cline and Mr. Reetz-Laiolo filed dueling lawsuits in federal court in San Francisco. In his complaint, Mr. Reetz-Laiolo, who is also a writer, claims that Ms. Cline plagiarized phrases and scene structures from him that appear in “The Girls,” and used spyware to read his email and other personal documents. Ms. Cline’s publisher and Scott Rudin Productions, which optioned the screen rights to “The Girls,” are also named in the suit, which seeks unspecified damages.

Ms. Cline’s countersuit calls the claims “ludicrous,” and says that the suit was motivated by jealousy, and marks the culmination of Mr. Reetz-Laiolo’s continuing effort to threaten and intimidate her into paying him a large sum of money.