In 1935, the mammoth retailer Marshall Field’s was bleeding cash. And so the company’s board of directors decided to bring in a man who had established a name for himself as a corporate makeover guru — James O. McKinsey.

McKinsey, sporting his signature blend of accounting, law, and management expertise, studied Marshall Field’s books for a few months and recommended that it liquidate its wholesale division, close its less profitable stores, and cut 1,200 jobs. Eager to make sure the transformation was a success, McKinsey put himself in charge of implementing the changes — a restructuring that came to be known as “McKinsey’s Purge.”

“Mac” didn’t survive his first foray into actually running a company — he died in 1937 from pneumonia. But the power of “The Firm” (as McKinsey insiders call it) to shape companies and governments has only grown stronger in the nearly one hundred years since its founding. Today, McKinsey & Company is a global powerhouse employing thirty thousand consultants in sixty-five countries and boasts alumni in the C-suites of more than seventy S&P 500 companies. Its clients include nine out of ten of the world’s biggest companies, and it has the ear of elected officials, royals, and demagogues the world over.