Steele had known Russia as a young spy, arriving in Moscow as a 26-year-old with his new wife and thin diplomatic cover in 1990. For nearly three years as a secret agent in enemy territory, he lived through the waning days of perestroika and witnessed the tumultuous disintegration of the Soviet Union under Boris Yeltsin’s mercurial and often boozy leadership. The K.G.B. was onto him almost from the start: he inhabited the spy’s uncertain life, where at any moment the lurking menace could turn into genuine danger. Yet even at the tail end of his peripatetic career at the service, Russia, the battleground of his youth, was still in his blood and on his operational mind: from 2004 to 2009 he headed M.I.6’s Russia Station, the London deskman directing Her Majesty’s covert penetration of Putin’s resurgent motherland.

And so, as Steele threw himself into his new mission, he could count on an army of sources whose loyalty and information he had bought and paid for over the years. There was no safe way he could return to Russia to do the actual digging; the vengeful F.S.B. would be watching him closely. But no doubt he had a working relationship with knowledgeable contacts in London and elsewhere in the West, from angry émigrés to wheeling-and-dealing oligarchs always eager to curry favor with a man with ties to the Secret Service, to political dissidents with well-honed axes to grind. And, perhaps most promising of all, he had access to the networks of well-placed Joes—to use the jargon of his former profession—he’d directed from his desk at London Station, assets who had their eyes and ears on the ground in Russia.

How good were these sources? Consider what Steele would write in the memos he filed with Simpson: Source A—to use the careful nomenclature of his dossier—was “a senior Russian Foreign Ministry figure.” Source B was “a former top level intelligence officer still active in the Kremlin.” And both of these insiders, after “speaking to a trusted compatriot,” would claim that the Kremlin had spent years getting its hooks into Donald Trump.

Source E was “an ethnic Russian” and “close associate of Republican US presidential candidate Donald Trump.”

This individual proved to be a treasure trove of information. “Speaking in confidence to a compatriot,” the talkative Source E “admitted there was a well-developed conspiracy of cooperation between them [the Trump campaign] and the Russian leadership.” Then this: “The Russian regime had been behind the recent leak of embarrassing e-mail messages, emanating from the Democratic National Committee (DNC) to the WikiLeaks platform.” And finally: “In return the Trump team had agreed to sideline Russian intervention in Ukraine as a campaign issue and to raise US/NATO defense commitments in the Baltic and Eastern Europe to deflect attention away from Ukraine.”

Then there was Source D, “a close associate of Trump who had organized and managed his recent trips to Moscow,” and Source F, “a female staffer” at the Moscow Ritz-Carlton hotel, who was co-opted into the network by an Orbis “ethnic Russian operative” working hand in hand with the loquacious Trump insider, Source E.

These two sources told quite a lurid story, the now infamous “golden showers” allegation, which, according to the dossier, was corroborated by others in his alphabet list of assets. It was an evening’s entertainment, Steele, the old Russian hand, must have suspected, that had to have been produced by the ever helpful F.S.B. And since it was typical of Moscow Center’s handwriting to have the suite wired up for sound and video (the hotel’s Web site, with unintentional irony, boasts of its “cutting edge technological amenities”), Steele apparently began to suspect that locked in a Kremlin safe was a hell of a video, as well as photographs.

Steele’s growing file must have left his mind cluttered with new doubts, new suspicions. And now, as he continued his chase, a sense of alarm hovered about the former spy. If Steele’s sources were right, Putin had up his sleeve kompromat—Moscow Center’s gleeful word for compromising material—that would make the Access Hollywood exchange between Trump and Billy Bush seem, as Trump insisted, as banal as “locker-room talk.” Steele could only imagine how and when the Russians might try to use it.

The Greater Good

What should he do? Steele dutifully filed his first incendiary report with Fusion on June 20, but was this the end of his responsibilities? He knew that what he had unearthed, he’d say in his anonymous conversation with Mother Jones, “was something of huge significance, way above party politics.” Yet was it simply a vanity to think that a retired spy had to take it on his shoulders to save the world? And what about his contractual agreement with Simpson? Could the company sue, he no doubt wondered, if he disseminated information he’d collected on its dime?