“I want to write a movie for Jack.”

“What kind of movie?”

“A detective movie. Maybe Jane Fonda for the blonde.”

“What’s it about?”

“Los Angeles. In the ’30s. Before the war.”

“What happens?”

“I don’t know. That’s all I know.”

Thus spake screenwriter Robert Towne to his then-girlfriend Julie Payne one night in 1970. And therein lies the seeds not only of “Chinatown” (1974), one of the greatest dark fables America has ever told about itself, but the legend of Towne himself, the most celebrated screenwriter of the New Hollywood era of the 1970s and a shadowy, mercurial figure behind the titles.

Towne, who died Tuesday in Los Angeles at 89, won a single Oscar, for writing “that movie for Jack [Nicholson]” as Sam Wasson recounts in his 2020 book “The Big Goodbye: Chinatown and the Last Years of Hollywood.” But his writing and his influence stretched far beyond “Chinatown” or the other movies on which he received credit — movies like “The Last Detail” (1973), “Shampoo” (1975), “The Firm” (1993) and the first two “Mission: Impossible” films (1996 and 2000).

A storied script doctor from the 1970s into the new century, Towne was the man you called in when your screenplay had painted itself into a corner and you needed a genius to get you out. Because he took payment but rarely credit for his surgical efforts, we owe Towne more than we realize. The heartbreaking scene between Don Corleone (Marlon Brando) and his son Michael (Al Pacino) at the end of “The Godfather” (1972)? Towne wrote it (and Francis Ford Coppola thanked him for it in his Oscar acceptance speech). The scene in “Bonnie and Clyde” (1967) where the gang kidnaps a man (the young Gene Wilder) who turns out to be a mortician and the air suddenly gets sucked out of the movie’s high spirits? Towne moved it earlier in the movie and rewrote it to cast the necessary pall of doom over the title couple.

“The Parallax View” (1974), “The Missouri Breaks” (1976), “Marathon Man” (1976), “Heaven Can Wait” (1978), “Reds” (1981), “Fatal Attraction” (1987), “Crimson Tide” (1995), “Armageddon” (1998) — all undercover Robert Towne rewrite jobs. Said a producer who worked with him on “The Last Detail,” according to Peter Biskind’s 1999 book, “Easy Riders, Raging Bulls”: “He had this ability … to leave a sense of moisture on the page, as if he just breathed on it in some way. There was always something that jostled your sensibilities … the feeling that something accidental and true to the life of a human being had happened there.”

The irony is that “Chinatown,” the movie that cemented Towne’s status among his peers and in pop-culture history, had its ending rewritten by director Roman Polanski over the screenwriter’s vocal protests. Urged by Payne to research the dirt behind Los Angeles’s founding, Towne wove a historic civic crime — the theft of the city’s water in the 1930s — into his detective story, and he compounded the sin by making John Huston’s villainous businessman Noah Cross an incestuous monster who’d fathered a child by his own daughter Evelyn (Faye Dunaway). In Towne’s version, Evelyn shoots her father dead. In Polanski’s film, Evelyn dies, evil triumphs and the rich get away with it. “Forget it, Jake. It’s Chinatown.”

Flashback to a few years earlier, when Towne was pitching the script to producer Robert Evans. According to Wasson’s book, Evans said, “It’s set in Chinatown?” Towne replied, “No. Chinatown is a state of mind.”

Exactly. Without Polanski’s ending, the film might not have lasted, might not continue to resonate right up to this very week in American history. But without Towne’s understanding of that “state of mind” — the murk we can’t see through, the hidden connections we’ll never fathom — “Chinatown” might never have existed in the first place.

There are darker sides to his story. Film historians like Biskind and Wasson have documented how cocaine fueled the work and destroyed the lives of not just Towne but many of his New Hollywood peers. The four movies he directed — “Personal Best” (1982), “Tequila Sunrise” (1988), “Without Limits” (1998) and a brave, foolhardy adaptation of John Fante’s 1939 L.A. novel “Ask the Dust” (2006) — have their glories but also their self-indulgences. And Towne’s secretive writing collaborations with former roommate and lifelong friend Edward Taylor, revealed at length in Wasson’s book, remain to be clarified. (Taylor styled himself as a “consultant” to the few who knew about the unpaid and uncredited relationship, but his input on the story structure of “Chinatown” and other Towne scripts may have been substantial. A subject for further research.)

And yet, in his ego and his talent, his idealism and soul-searching doubt, Towne was at least as much the author of the films he wrote as the men who directed them. And he was as much an architect of America’s post-’60s cultural “state of mind” — impassioned, conspiratorial, hopeful, damned — as anyone could claim to be. If movies, at their best, are stories we tell about ourselves, Robert Towne told the truth.

Ty Burr is the author of the movie recommendation newsletter Ty Burr’s Watch List at tyburrswatchlist.com.