Paul McKerrow was an all-American boy. Raised in Helena, Montana, he was the quarterback for his high-school football team, which is as close to being idolised as many small-town Americans come.

He was also his class president, the ­valedictorian of his year in 1985 and voted most likely to succeed by his classmates. He was tall and ruggedly good-looking. McKerrow, in short, had it made and great things were expected of him.

So it was with some trepidation that McKerrow recently attended his 20-year high-school reunion as Kimberly Reed, a lesbian, New York-based film-maker who had had gender reassignment to become a woman.

"It was very emotional. I wanted it to go smoothly. People get freaked out enough by going to their high-school reunion. But having a new gender is a big surprise for a lot of people," Reed said.

Yet Reed found that her worries were unfounded. Defying the preconceptions that surround many people's views of small-town America, she was welcomed home with open arms. "It has been really great. It really was easy. That became the surprise," she said.

Reed has now made a documentary about her story, which has become a major hit on the American film festival circuit. The movie, called Prodigal Sons, is getting its cinematic release in New York in two weeks and has already won plaudits from the critics for its painful and honest depiction of Reed's experience, as well as that of her ­family, ­especially her brother, Marc. It has been called "exceptional" by the Village Voice and "superb" by the San Francisco Chronicle and has won nine awards.

Certainly, Reed's story is fertile ­territory for exploration. She said she had always felt uncomfortable growing up as a boy, despite the fact she clearly excelled at traditional male activities such as American football. In the time before the internet, she used to spend many hours in Helena's libraries looking for information that would help explain her feelings. "I really tried to suppress it during my school years. But it was something that I knew was going on for as long as I can remember," she said.

Reed finally went to college in San Francisco. In the more liberated environment of the West Coast, she came to decide that she was a woman born in a man's body. Gradually, her identity as a woman began to emerge and she started spending part of her time as Paul and part as Kim. Eventually, Kim came out fully and she underwent gender reassignment and became a lesbian, living and working in the film industry as a woman. When Kim returned to Montana, first for the death of her father and then for her high-school reunion, it seemed like a natural subject for a documentary.

But Prodigal Sons is not just about Kim's shifting identity. In its examination of her family and the changes that time can bring, it also tells the heart-rending and surprising story of Marc Kim's brother. Marc, who was adopted, suffered serious brain damage in a car accident when he was 21. He also attended the 20th high-school reunion (although older than Kim, he was held back a year) with a different identity; one caused by the personality changes that the crash caused, leaving him reliant on medication and prone to temper tantrums.

But perhaps the most stunning identity change charted in the film is Marc's quest to find his real parents. To the amazement of everybody, Marc discovered his real mother was the daughter of film-maker Orson Welles and actress Rita Hayworth. That discovery fundamentally altered the way the two siblings perceived each other. "Marc always envied my genes. Now I am envying his," laughed Kim.

Such revelations make the movie surprisingly universal in its appeal. Though the film's two main characters undergo extreme changes in identity, Kim said anyone can find common ground with their experiences. "We all grow up and become someone new in one way or another. My family's experience is a bit more dramatic than usual, but it happens to all of us," she said. She even said that returning home and making the movie had allowed her to appreciate the fact she was born a boy, rather than rejecting her previous identity altogether. "I am glad that I had that upbringing. It taught me that I can do anything," she said.