Sharing more of the details, Dolores Chaplin, granddaughter of Charlie Chaplin, tells China Daily that the letter seemed quite mysterious, as her grandfather received millions of letters from fans all over the world, keeping them stacked in piles in special offices.

"But this one was locked in a drawer with nothing else," she continues. "The letter said, 'I'm not asking for money. I'm not trying to blackmail you. But if you want to know more about where you come from, please contact me.' I don't think my grandfather ever contacted him."

Her father, Michael Chaplin, was the first person to develop a passion for uncovering the story behind the letter and to propel the project forward.

"My father is a very secretive person who doesn't speak much about his own father. We have to ask him for stories, and then he'll share all these anecdotes and different memories from his childhood," explains Dolores, also a producer of the documentary.

As the second of eight children of Charlie Chaplin and O'Neill, Michael Chaplin grew up in Switzerland. In 1952, Charlie Chaplin had traveled to Europe to promote his film Limelight when he learned that he had been exiled from the United States as a victim of McCarthyism. On the advice of a relative, he decided to settle in Vevey, a small town situated on the eastern shore of Lake Geneva during his last 25 years.

For Dolores Chaplin, the documentary marking a journey of seeking roots is "healing" for her father and other family members.