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One of cinema's most epic plot twists happened in George Lucas' movie "The Empire Strikes Back" back in 1980.

At the end of the "Star Wars" franchise film, Darth Vader tells Luke Skywalker as he's fighting the young Jedi on a space bridge, “I am your father."

According to Skywalker himself, Mark Hamill, the line was so secretive that "only [director] Irvin Kershner, [creator] George Lucas & I knew."

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“The cast & crew first learned of it when they saw the finished film,” Hamill revealed in the tweet responding to a fan. “When we shot it, Vader's line was ‘You don't know the truth, Obi-Wan killed your father.’ Only Irvin Kershner, George Lucas & I knew what would be dubbed in later. Agony keeping that secret for over a year!”

"Star Wars" fans were stunned to learn this new bit of movie trivia.

"This might be my favorite movie trivia fact every (sic), so great!" one fan wrote.

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Another replied, "I will make it my life mission to make sure my child does not know that twist until we watch the film. It is the greatest film twist of all time."

Hamill has previously spoken about the plot twist. In 2017, on "The Graham Norton Show," the actor said that Kershner took him aside and warned him to never tell a soul.

"The director, Irvin Kershner, took me aside, he came to my dressing room actually, and he said, 'I'm gonna tell you something. I know it. George Lucas knows it, and when I tell you, you'll know it. But if it leaks, we'll know it was you,'" Hamill revealed. "So I said, 'What? What?' and he handed me the piece of paper that said, 'I am your father.'"

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"I was shocked," Hamill described. "I said, 'Is that true?' and he said, 'Well, you search your feelings, and we're gonna play it like yeah, it is true!' I said, 'Oh my gosh, Dad Vader!'"