I fear I may be too tall for you, Mr. Tracy," Katharine Hepburn (5'9" in heels) declared in crisp New England tones when she was introduced to her costar by director Joe Mankiewicz on the MGM lot. "Don't worry," said Mankiewicz, in defense of the 5'10" Spencer Tracy. "He'll cut you down to size."

He bespoke more than he knew. In the nearly 30 years that Hepburn and Tracy spent together—starring together in nine films—there would be much shifting in shape. When they met, the 41-year-old actor was a hard drinker and a guilt-ridden Catholic, old-fashioned enough to be scandalized by the brusque actress in rumpled trousers. She, then 33, was the liberal Yankee blue-blood divorcée (she had left businessman Ludlow Ogden Smith after four years) who was disinclined to give a damn what anyone—Mr. Tracy included—might think. Within a week after starting Woman of the Year, in 1941, they were calling each other Spence and Kate—and discovering a once-in-a-lifetime magic.

"At lunchtime they'd just meet and sit on a bench on the lot," Gene Kelly once recalled. "They'd hold hands and talk—and everybody left them alone in their little private world."

Alone but not carefree. Tracy's wife, Louise, the mother of his two children, suffered in silence. Tracy, already living apart from his family, settled into a cottage near Hepburn's Beverly Hills house. Outside work, the two were careful not to be seen publicly or photographed together, and Louise never asked for a divorce, the only way that Tracy would end his marriage. After Tracy's fatal heart attack in 1967, Hepburn called Louise (who died in 1983) to see if a peace could be reached between them. "I thought you were a rumor," Mrs. Tracy responded.

The rebuff stung, but Hepburn's love had given her thick skin. Tracy's displays of affection were gruff—and not helped by his battles with alcohol and depression. In front of visitors, he often sat in judgment as Hepburn brandished opinions. On one occasion remembered by director Stanley Kramer, she spoke of Russian missiles. "So that's another one of those subjects you know all about," Tracy growled.

"Oh, Spencuh," Hepburn responded.

"Why do you always talk like you have a feather up your ass?" Tracy shot back. At which—amazingly—the indomitable Miss Hepburn giggled.

Author Martin Gottfried, who was friendly with Hepburn in the '80s, confirms the sometimes abusive nature of the relationship. "She talked about his drinking and the fact that he hit her, which stuns me," he recalls. "I'm not telling you she justifies violence, but she certainly forgave it in Spencer's case."

In the end, the size they assumed was not his or hers but uniquely theirs. "I loved Spencer Tracy," Hepburn wrote in her 1991 autobiography, Me. "I would have done anything for him."