On Tuesday, two of the President’s most prolific accusers plan to disrupt the narrative by telling their own story. Glenn Simpson and Peter Fritsch, the co-founders of the Washington-based private-investigative firm Fusion GPS, which has mined deep veins of muck on Trump for years, at the behest of his various political enemies, will try to throw the book at Trump with the publication of “Crime in Progress: Inside the Steele Dossier and the Fusion GPS Investigation of Donald Trump.”

Fusion was the firm that hired the former British spy Christopher Steele to research Trump’s ties to Russia during the 2016 campaign. After nearly three years without a word from Steele, while the so-called pee tape and his other sensational findings sparked furious controversy, the former M.I.6 spy speaks directly and on the record about his own part for the first time in the book, an advance copy of which was given to The New Yorker.

Whether Simpson and Fritsch’s score-settling, tell-all account will change any minds remains to be seen, but they present a mountain of evidence that Trump’s dealings with corrupt foreign players—particularly those from the former Soviet Union—are both real and go back decades. Steele’s dossier has been debated, denounced, derided, and occasionally defended almost since the moment it was first published, in January, 2017, by BuzzFeed News, against Steele’s wishes. Although Carl Bernstein helped to break the news of its existence on CNN, his friend and Watergate-reporting partner Bob Woodward dismissed it almost instantly as “garbage.” During impeachment-hearing testimony last week, the former White House national-security adviser Fiona Hill, one of America’s foremost experts on Russia and a professional acquaintance of Steele’s, described the dossier as “a rabbit hole” and suggested that Steele may have been “played.” But the authors defend Steele’s work, and their own, arguing that it has proved “strikingly right.”

As the authors tell it, they became obsessed with Trump almost accidentally. Their involvement in his campaign began as a business proposition. In the past, they had worked mostly for corporate clients, but in 2012 they had also done some political-opposition research on the Republican Presidential nominee, Mitt Romney. (They declined to disclose their client.) So, in 2015, as Trump gained momentum, but before he clinched the nomination, Simpson and Fritsch again decided to look for political work. After firing off a quick e-mail to a big conservative donor they knew who disliked Trump, they were hired. They don’t identify that donor but note, helpfully, that he arranged for them to contract their opposition-research assignment through the Washington Free Beacon, a conservative Web site known to be funded by Paul Singer, a New York hedge-fund magnate. Once Trump secured the nomination, however, the G.O.P. donor fled.

At that point, Fusion switched clients and political parties, pitching its services to Marc Elias, the lawyer for the D.N.C. and Hillary Clinton’s Presidential campaign. Clinton’s identity, too, was kept hidden, in this case behind the screen of Elias’s law firm, Perkins Coie. In the beginning, Clinton’s identity was also hidden from Steele, who knew only that Fusion was hiring him in the late spring of 2016, as a contractor, to investigate the tangled web of Trump’s ties to Russia for an unknown patron. Contrary to the conspiracy theories that the right later spread, Simpson and Fritsch write that they never met or spoke with Clinton. “As far as Fusion knew, Clinton herself had no idea who they were. To this day, no one in the company has ever met or spoken to her,” the book reads. As I reported, although Steele went to the F.B.I. with his findings out of a sense of duty and, by the late summer of 2016, knew that the F.B.I. was seriously investigating Trump’s Russian ties, the communication channels were so siloed that the Clinton campaign was unaware of these facts. Far from conspiring in a plot, the Clinton team had no hard evidence that the F.B.I. was investigating its opponent, even as its own opposition researcher was feeding dirt to the F.B.I. As one top Clinton campaign official told me when I wrote about Steele, “If I’d known the F.B.I. was investigating Trump, I would have been shouting it from the rooftops!”