Out of the blue in the fall of 2010, a blogger asked Jane Mayer, a writer with The New Yorker, how she felt about the private investigator who was digging into her background. Ms. Mayer thought the idea was a joke, she said this week. At a Christmas party a few months later, she ran into a former reporter who had been asked about helping with an investigation into another reporter on behalf of two conservative billionaires.

“The reporter had written a story they disliked,” Ms. Mayer recounts in “Dark Money: The Hidden History of the Billionaires Behind the Rise of the Radical Right,” out this month from Doubleday. Her acquaintance told her, “‘It occurred to me afterward that the reporter they wanted to investigate might be you.’”

As it happened, Ms. Mayer had published a major story in the magazine that August about the brothers David and Charles Koch, and their role in cultivating the power of the Tea Party movement in 2010. Using a network of nonprofits and other donors, they had provided essential financial support for the political voices that have held sway in Republican politics since 2011. “Dark Money” chronicles the vast sums of money from the Koch brothers and other wealthy conservatives that have helped shape public dialogue in opposition to Democratic positions on climate change, the Affordable Care Act and tax policy.

Ms. Mayer began to take the rumored investigation seriously when she heard from her New Yorker editor that she was going to be accused — falsely — of plagiarism, stealing the work of other writers. A dossier of her supposed plagiarism had been provided to reporters at The New York Post and The Daily Caller, but the smears collapsed when the writers who were the purported victims made statements saying that it was nonsense, and that there had been no plagiarism whatsoever. Indeed, as one noted, Ms. Mayer had plainly credited his writing — though this was not mentioned in the bill of particulars that was passed around.