WATCH: The Number One Moment of Kate Middleton's Tour of North America

Though Prince William and Kate Middleton's tour of North America was filled with glamorous moments such as attending the BAFTA Brits to Watch gala in L.A. and playing polo in front of A-listers like Zoe Saldana and Rosario Dawson, it was a touching moment between Kate and a 6-year-old cancer-stricken girl that turned out to be the standout moment of the couple's historic visit.

The images of Duchess Kate hugging Diamond Ann Marshall, who suffers from stage four cancer, at the airport in Alberta were displayed all around the world, the unscripted moment of the two hugging touching millions.

Us Weekly recently obtained the catalyst for the once-in-a-lifetime moment -- the letter Diamond wrote to "Princess Kate" -- which made its way into the hands of the royals through the Alberta Chapter of the Children's Wish Foundation of Canada. The hand-written note reads:

"Dear Princess Kate,

My name is Diamond Ann. I am six. I was named after Princess Diana. My Mommy Memory is in heaven with her. I have cancr [sic]. I spend a lot of time at the hospital. I watched you get married from my bed there. You looked pretty.

I like playing Princess dress up. My favorite princess is Aurora. Who is yours? I would really like to meet you. Do you wait [sic] to meet me too? Thank you for reading my letter.

Love,

Diamond"

Diamond's father Lyall Marshall tells the magazine that the brief meeting was a "dream come true" for his daughter.

"She had always wanted to meet a real princess," Lyall tells Us Weekly. "They got along very well. At first Di ran over and hugged Kate's legs and then ran back to us after handing over the flowers, but then Kate kneeled down and made her feel welcome. They talked for a few minutes. [Kate] told us the flowers were beautiful. The Duchess was very kind to her, very sweet and genuine."

Check out the video to see the heart-warming moment.