(FORTUNE Magazine) – The best way to describe Peter Fisher is that he's one of those behind-the-scenes guys who keep Wall Street from coming apart at the seams--which it almost does every few years. As the recently departed undersecretary for domestic finance at the Treasury, believe you me, Peter Fisher is known to every fiscal power broker from D.C. to Delhi. He's an unusual guy. For one, he gets visibly excited talking about financial policy. And for another, he's been pretty good at implementing it. He's also obsessed with the business of risk.

Now Fisher has hung up his government scepter to join bond house BlackRock. (Fisher was on everyone's short list to head the New York Fed or lead the New York Stock Exchange.) It's a cashing-in of chips, partly. As a long-standing public servant, Fisher--at 47, a friendly cerebral type--was earning peanuts in government and is looking for his first real payday. It's also the reverse of the more tried-and-true New York to Washington route (whereby Goldman Sachs partner makes fortune on Wall Street and then goes to D.C. to work in government).

But as I said, Fisher is different. He's a bit like his erstwhile boss, former Treasury Secretary Paul O'Neill, in that he's unafraid to take iconoclastic or unpopular stands (though he's much more politic about articulating his positions). While Fisher is wary of speaking on the record about O'Neill's latest tempest, he isn't shy about pointing out that he voted against bailing out the airlines after 9/11. And he angered many a bond trader by doing away with the 30-year bond.

Not that Fisher came to any of those decisions lightly. We are talking about a major deep-think wonk here. He throws out lines like this in the course of a conversation: "Risk is deviation from objective. Anybody who doesn't understand that is lost." Fisher comes by that sort of talk naturally. "I'm kind of the black sheep of my family," he tells me. "My great-grandfather, my grandfather, and father all argued cases before the Supreme Court." (His great-grandfather was also Secretary of the Interior under Taft.) Fisher, who's dyslexic, grew up in Cambridge, where his father, Roger Fisher, taught law at Harvard.

Fisher went to Concord Academy--where he became friendly with Caroline Kennedy--and then to Harvard College and Harvard Law. He took a job at the New York Fed, where he became a favorite of Fed chief William McDonough. There Fisher made his first mark, resolving the collapse of Long-Term Capital by organizing a $3.6 billion bailout from Wall Street's biggest institutions.

Fisher, a Democrat, joined Bush's Treasury in August 2001 and was immediately put to the test by 9/11. With the financial markets literally blown to bits, it was Fisher's job to help restore order and calm, and, most important, get the markets up and running. He was roundly lauded for doing just that. Fisher also drew kudos for turning down Bob Rubin's suggestion that he call ratings agencies and encourage them to hold off on downgrading Enron's debt so that creditors could attempt a rescue.

So what's with Fisher taking the job at BlackRock? Well, Fisher will act as an uberconsultant-salesman, offering up his and the firm's expertise in managing customers' asset-liability risk. You might expect him to land a higher-profile job, but as Fisher tells it, the more he spoke with BlackRock CEO Larry Fink, the better the fit felt. As a money manager of the $3 trillion fixed-income portfolio of the U.S. government for the past two years, Fisher says he's comfortable at BlackRock, which now manages more than $300 billion of mostly debt and cash. And then there's the risk thing: "I am so happy to be at a place that is practicing what I have been preaching for years," he says, sitting in his brand-new BlackRock office. "The whole key is understanding risk. Risk management isn't avoidance. But so many companies just don't understand what they have taken on." Fisher is the perfect guy to explain it to them.

ANDY SERWER, editor at large of FORTUNE, can be reached at aserwer@fortunemail.com. Read him online in Street Life on fortune.com and watch him on CNN's American Morning and In the Money.