Katharine Hepburn couldn’t deny her feelings when she was introduced to Spencer Tracy on the set of 1942’s "Woman of the Year."

"It was one of those love-at-first-sight things," the late star’s pal and biographer Christopher Andersen recently told Closer Weekly.

"I don’t think she admired him tremendously as an actor… [but] there was an instant attraction on her part," he said.

The outlet noted that Hepburn tried to act nonchalant about the encounter and even made jokes about Tracy's height. However, a romance blossomed between the pair and lasted until the actor’s death in 1967 at age 67 from a heart attack.

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Actors Spencer Tracy as Sam Craig and Katharine Hepburn as Tess Harding in the romantic comedy film 'Woman of the Year', 1942.  

Actors Spencer Tracy as Sam Craig and Katharine Hepburn as Tess Harding in the romantic comedy film 'Woman of the Year', 1942.   (Photo by Silver Screen Collection/Getty Images)

Andersen has previously written about his friendship with the star in a book titled "An Affair to Remember: The Remarkable Love Story of Katharine Hepburn and Spencer Tracy." Andersen told the outlet that despite Tracy’s deep feelings for Hepburn, he couldn’t bring himself to divorce his wife Louise.

The couple was married from 1923 until his death. They shared two children. According to the outlet, Tracy reserved the weekends for his family and spent the weekdays with Hepburn for the last 26 years of his life.

"Everyone thinks it was because he was a religious Catholic, but wasn’t it," Andersen alleged on why Tracy and Louise never divorced. "He felt that Louise was like a saint. He didn’t want to put her through any unnecessary scandal or pain."

Andersen claimed that Hepburn told him that not only did she accept the arrangement, but she also had no desire to marry Tracy or have children with him.

Hepburn passed away in 2003 at age 96. She was married once, to Ludlow Ogden Smith, a Philadelphia socialite, from 1928 until 1941.

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Spencer Tracy with his wife Louise Treadwell Tracy.

Spencer Tracy with his wife Louise Treadwell Tracy. (Getty Images)

In her memoir, Hepburn admitted she neglected the marriage for an acting career in New York.

"What the hell would I have done without Luddy – my protector?" she wrote, as quoted by the Chicago Tribune. "I would have been frightened away from this big city, and I would have shriveled up and died. And Luddy – all he wanted was me, and of course, all I wanted was to be a great big hit star in the movies... I am horrified at what an absolute pig I was."

Andersen said Hepburn "had a streak of loyalty and caring, which helped in her relationship" with Tracy.

Even as Tracy battled alcoholism, Hepburn remained by his side, Andersen claimed.

"On set, she was a mother hen to Tracy – getting him pills, milk for his ulcer, fetching coffee, sitting at his feet and gazing at him adoringly, praising him to the skies," he alleged.

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Spencer Tracy passed away in 1967 at age 67.

Spencer Tracy passed away in 1967 at age 67. (Getty Images)

Hepburn and Tracy appeared in nine movies together. According to the Chicago Tribune, Tracy and Louise had long been separated, but they never divorced. When Tracy became ill, Hepburn took time off to help care for him, the outlet noted.

Andersen claimed Hepburn was with Tracy the night he died. However, she didn’t attend the funeral to avoid a media spectacle that would have impacted a grieving Louise.

"She went to the mortuary," said Andersen. "She watched while they took the coffin out of the mortuary and put it into the hearse. She said goodbye to him there."

"They were kindred spirits – soulmates in a way," he shared.

After releasing her memoir in 1991, Hepburn maintained a private life until her death. But in her book, the actress did address her relationship with Tracy in a chapter titled "Love."

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Spencer Tracy and Katharine Hepburn were partners in life and in work for many years.

Spencer Tracy and Katharine Hepburn were partners in life and in work for many years. (Photo by Mondadori via Getty Images)

"I have no idea how Spence felt about me," she wrote. "I can only say, I think that if he hadn’t liked me, he wouldn’t have hung around. As simple as that. He wouldn’t talk about it, and I didn’t talk about it. We just passed 27 years together in what was to be absolute bliss. It is called LOVE."

Louise passed away in 1983 at age 87. At the time of her death, The New York Times reported she never discussed the cause of her separation from Tracy. However, Tracy’s biographer Charles Higham wrote that "she shared the pain of her husband’s final days with his friend, the actress Katharine Hepburn."