At unpredictable and unpleasantly irregular intervals, a beep resounds through the office of a tired-looking man on the 13th floor of a building in New York’s financial district. The sound is loud and distracting and can fray the nerves. No merry jingles, no downloaded ringtones, just the beep of Tim Geithner’s cell phone.

As president of the Federal Reserve Bank of New York, Geithner, at least at this point in early April, is the man of the moment. Credit-crunched investment bankers are calling to withdraw funds from the discount window, which the Fed uses to loan money directly to banks. Nosy politicians are trolling for scapegoats. Journalists are asking what will happen next.

Everyone is calling at once, and more often than not, their questions pertain to a singular event that rocked the financial markets, the Fed-financed bailout-cum-acquisition of Bear Stearns by J.P. Morgan Chase—a desperate move played out over four hellish days in mid-March, the most significant government intervention in the financial markets since the Great Depression.

Geithner was the central figure in that drama. It was Geithner’s Federal Reserve bank, not the Treasury, that came up with the $29 billion loan that made the deal possible or, more precisely, acceptable to J.P. Morgan. Geithner brought the parties together, hashed out the details, and demanded answers when things got shaky. It was a heady role for a noneconomist who has, to put it kindly, only on-the-job training in the financial markets and who relies on an A-list inner circle. Officially, his advisers include the board of the New York Fed, which counts several heads of financial institutions as members. Unofficially, he has built an impressive career with the help of a number of kingmakers, including some with a financial interest in the industry he oversees. (See a pop-up graphic showing Tim Geithner's unofficial advisors.)

It was in this office, right here, where the Bear deal was done. During that time and in the weeks after, Geithner was getting two hours of sleep a night, and he still looks it. You might even say that this youthful 46-year-old is starting to look his age. The sudden fame clearly unnerves Geithner, a quiet sort who is described by people who know him as shy. “He does not try to blow you away, to overwhelm you,” says Henry Kissinger, Geithner’s first boss.

The Bear bailout is an ordeal Geithner is loath to repeat, even though he knows he may have to. He’s already anticipating the regulatory shifts that must be implemented if the markets are to withstand further shocks. “We’re going to need to change a whole bunch of aspects of our financial system,” Geithner says, speaking quickly and leaning forward in his chair. “We should not have a system that’s this fragile, that causes this much risk to the economy.”

The reform process has started creaking forward, with a wide-ranging (and swiftly dismissed) series of proposals by Treasury Secretary Hank Paulson. Meanwhile, Geithner has begun sending teams of examiners to the major investment banks to pore over their books and risk-control policies. Since the Bear blowup, he has also been urging bankers to boost their capital levels.