Those of us who find the concept of “soul mates” corny and wistfully delusional may be defenseless against the intimate generosity on display in “Chris & Don: A Love Story.” This quiet, observant documentary, which opens Friday at Quad Cinema, charts the unique, lifelong relationship between the English expatriate writer Christopher Isherwood and his lifetime companion, the artist Don Bachardy, whom Isherwood first met on a beach in Santa Monica in 1952, when Mr. Bachardy was only 18.

Isherwood, who was 30 years older than his lover, had a keen interest in young men. It was what had lured him to the sexually liberated Weimar Berlin, where he dove headlong into the gay subculture. His ensuing experiences formed “The Berlin Stories,” the book that eventually made him famous through the stage and screen adaptations “I Am a Camera,” which later inspired “Cabaret.” But Isherwood also had a deeper yearning for a committed relationship. After fleeing Germany (with close friend W.H. Auden in tow) for New York and then Los Angeles and conducting a brief affair with the photographer William Caskey, he finally discovered what he was looking for in the teenage Mr. Bachardy.

“There’s a brilliant wide-openness about his mouse face, with its brown eyes and tooth gap and bristling crew-cut which effects everybody who sees him,” Isherwood wrote in his diary. “If one shall be like that at 40, one would be a saint.” The writer’s recollections, and a trove of photographs and old home movies, are all filmmakers Guido Santi and Tina Mascara have of the perspective of Isherwood, who died in 1986. Thus, much of the story is told by Mr. Bachardy, a sweet old gent whose speech retains the elegant traces of the well-schooled English accent he picked up from nearly 35 years with Isherwood.

It’s a fascinating study in the emergence of openly homosexual life in America. The Hollywood demimonde, which became the men’s natural habitat in the 1950s and ’60s, has become known for a generation of closeted icons. And as Mr. Bachardy notes, with a bit of self-amusement, he and Isherwood would often see men whom they had bedded at parties accompanied by their guileless wives. Once easily mistaken for father and son, the lovers had an uneasy balance at times. Isherwood commanded respect even from homophobes, and included among his friends such icons of art as Igor Stravinsky and Tennessee Williams. Mr. Bachardy was usually regarded as something of a male bimbo. He had to find a way to distinguish himself as meaningful outside of Isherwood’s extravagant shadow. And so he did. Mr. Bachardy took up life drawing and portrait painting, often using as his subjects the very movie stars he had once idolized from afar.

The elderly Mr. Bachardy makes a wonderful character. Still trim in a white sleeveless T-shirt and painter’s pants, his silver hair wispy above a face that retains a good bit of the chiseled features of his youth, he leads the camera around the home he shared with Isherwood, opening up boxes of memories with an emotional transparency that shows the enduring value of his commitment to Isherwood. It’s something lovely and rare.