The actress reached out to 10-year-old schoolboy Oliver Burton, who is battling cancer in his spine and may have just weeks to live, after learning he was unable to meet the real Queen at Buckingham Palace.
Dame Helen then stepped in to help make Burton's dream come true - she treated him to a limousine ride to see her London play The Audience, in which she portrays the monarch, and met him in character backstage at the Gielgud Theatre after the show.
The star, who introduced Burton to her corgi dog co-star and even pretended to knight him during the visit, tells The Sun on Sunday, "It was a pleasure and a privilege to meet such a brave young man."
Burton's dad James adds, "She stayed in character for the whole thing and had a butler. She was really lovely."