A lawsuit alleges that author Emma Cline plagiarized parts of her bestselling novel "The Girls" from an ex-boyfriend by spying on his email and other accounts.
Cline vehemently denies the claims and filed a countersuit.
She says the plagiarism allegations are the "ludicrous" acts of a man who is jealous of his ex's success.
The clashing lawsuits were filed Wednesday in federal court in San Francisco.
Chaz Reetz-Laiolo, Cline's former boyfriend, also names Penguin Random House in the lawsuit, saying the publishing house knowingly released plagiarized content when it published "The Girls."
The novel, released in June 2016, tells the story of a 14-year-old girl who gets involved in a dark 1960s cult that closely resembles the Charles Manson family.
The book made Cline a literary sensation and was on The New York Times' best-seller list for 12 weeks.