High-school quarterbacks, it's reasonable to assume, look upon their high-school reunions as moments of triumph. Oh, those romantic conquests! Oh, those hagiographic articles in the local press, those tailgate parties, those long-ago legends—let us now relive them! What could be more Friday Night Lights, therefore, than the 20th reunion of Helena, Montana's own Paul McKerrow, who was not only Helena High's starting quarterback and football co-captain but its class valedictorian and the student voted most likely to succeed? Those of us who languished on the sidelines of high-school life can only imagine such a return. But was the grand entrance we're imagining here really the stuff of local history? Or was it something else?

Because Paul McKerrow, it must be said, is no longer Paul McKerrow. He is, these days, Kimberly Reed, filmmaker, New Yorker, and transgender person. It was Kimberly who in 2005 strode into the 20th reunion for Paul, and who got light-headed enough to wear her name tag, which had her new name, and an old photo of her former self. It was Kimberly who in the months after the reunion made a very moving documentary about it, Prodigal Sons, to be released in theaters in late February by First Run Features and broadcast on the Sundance Channel in June.

Let's go back a little bit: Once, there was an upstanding ophthalmologist out West, who, with his wife, was unable to start a family. They adopted a bundle of joy, therefore, in 1966, and then, as is often the way with such things, they managed to conceive almost instantly, maybe even the very day their first child arrived home. When that second baby was born, at a nearby Army hospital, the doctors took one look and said, "Oh, it's a little baby girl!" And then, "Oh, wait a moment . . ." And thus began Paul's life of gender-related ironies. Just over a year later, a third son likewise arrived the old-fashioned biological way, and so the prodigal sons were assembled.

Marc, the adopted son, had problems almost immediately. Not given to concentrating, he was unable to settle down in preschool, and the educators encouraged the McKerrows to hold him back a year. Unfortunately for Marc, he then landed in the very same grade as his unadopted younger brother, Paul, the Golden Boy. "There was always this rivalry between us," Kimberly Reed says now, from the vantage point of her early forties, and this is understatement of the sort that only a woman could fashion to describe the behavior of men.

Since I am already committing the sin of journalism, let me pause to observe that by any barometer, Reed, despite being over six feet and having the lean physique of an athlete, is a stunning and beautiful woman, which is to say that she could pass, which is a political statement in certain redoubts of the transgender community. Kimberly Reed can pass, though the heroic journey of Paul McKerrow has made this less important to her, if relevant at all. She is blonde, modest; smiles easily. She has none of the stoic, determined, slightly overcalibrated femininity that I sometimes associate with women who have transitioned. On the contrary, Kim Reed is a little goofy, with a bit of the former straight-A student about her. If anything, she's what you would expect of a woman who grew up in Montana, whose grandparents worked the land, and whose extended family still does.

I met Reed at one of those artists' colonies, the one called Yaddo, in Saratoga Springs, New York. She was there crafting a screenplay while I was finishing my fifth novel, and I remember well the night she strode into the Linoleum Room looking for mail, because I thought, Extremely tall and very pretty and startlingly normal. Not another art-colony head case! We had some friends in common. So I was interested and intrigued when Reed volunteered, a night or two later, that she was going to show a small group of us her film in the drawing room. I don't know what I was expecting from Prodigal Sons, but certainly not the harrowing family drama that ensued, in which Reed's transition from male to female is only the first and most obvious layer.