Fred Turner did not need to look at financial statements to know McDonald's was in trouble. He could taste it.

The man who worked alongside founder Ray Kroc to turn McDonald's Corp. into a global colossus, Turner noticed when penny pinchers at corporate headquarters changed recipes to cut costs. So when McDonald's cheapened the famed "special sauce" on its flagship Big Mac sandwich, Turner knew.

But it wasn't until a new CEO brought him back from retirement 18 months ago to help lead a turnaround at McDonald's that the now 71-year-old Turner learned just how deep the trouble ran.

McDonald's had lost the recipe to the special sauce.

In the annals of marketing, the Big Mac special sauce has a sizzle like Coke's secret formula or Kentucky Fried Chicken's secret blend of 11 herbs and spices. To people inside McDonald's, losing the special sauce meant a loss of connection with the company's very roots.

It didn't take Turner long to find it.

He recalled the name of a California supplier who had helped develop the sauce 36 years ago. The supplier recovered the recipe.

"It's time to make a hamburger the way we used to make a hamburger," Turner said to the 25 restaurant owners on McDonald's Food Improvement Team.

The return of the special sauce is one of hundreds of changes, big and small, that McDonald's has made to execute one of the most stunning turnarounds in corporate history.

It is a recovery that has brought customers back to the restaurants and brought the company's fortunes back from the brink. The recovery has proven stronger than most experts expected. It has lasted longer than many predicted.

But now the McDonald's recovery must stand its toughest test. It must survive twin shocks.

First came the fatal April 19 heart attack of Jim Cantalupo, 60, the executive who led the turnaround. Two weeks later came the news that his successor, 43-year-old Charlie Bell, has colon cancer. The cancer has spread to Bell's lymph nodes.

Even under normal circumstances, the effort to maintain momentum would be a tall order for McDonald's. After all, the system of 30,000 restaurants worldwide had just become familiar with the taste of success after years of sour results.

Before Cantalupo returned from retirement in late December 2002, McDonald's was in tatters. Existing restaurants were losing sales, and the traditional McDonald's response--building new ones to create growth--was sapping money and distracting management.

The company's stock price had fallen 60 percent in three years. Debt climbed, and profits fell.

McDonald's even had lost its marketing vision. That was evident in the ads for the Arch Deluxe sandwich, which emphasized how children disliked the taste. One even showed Ronald McDonald frowning.

By December 2002, the McDonald's board of directors had seen enough. Urged on by Turner and former Chief Executive Michael Quinlan, the board ousted then-CEO Jack Greenberg and brought Cantalupo back from retirement.

The turnaround that Cantalupo and Bell led has Ronald McDonald smiling again. Sales are growing at a double-digit rate, and restaurant owners are posting operating profits they haven't seen in 15 years. Menu offerings, from vintage Big Macs to new premium salads, have scored well with customers.

But the health shocks in the company's executive suite--Cantalupo's death and Bell's cancer--mean the story of McDonald's turnaround must be told in two parts.