Nine years after Merle Haggard’s death at 79, Ethan Hawke has organized a public wake, and it’s a humdinger. A biography by way of California road trip and recording-studio extravaganza, Highway 99: A Double Album is, at its soul-stirring core, a gathering of several dozen musicians singing the country great’s praises, mainly by singing his songs.

In a sense Hawke is doing here what he did a few years ago in The Last Movie Stars, his docuseries portrait of Paul Newman and Joanne Woodward: He’s stirring up a conversation about a legendary performer. That earlier doc felt more theoretical for two key reasons: Hawke’s searching convos were confined to pandemic-era Zoom, and the subject was acting as opposed to music, with its more visceral immediacy. But while they’re very different experiences, the three-and-a-quarter-hour Highway 99 is fueled by the same exuberance that propelled Movie Stars.

The double album consists of 26 tracks, intimate performances of songs from Haggard’s staggering body of work. The film is divided into two parts, with a theater-friendly 15-minute intermission (with extra music packed in there, too), and the song interpreters include Lucinda Williams, Dwight Yoakam, Gillian Welch, John Doe, Steve Earle, Los Lobos and Valerie June. Every one of the new performances reveals how finely the songs have aged. But it’s in the moments just after each song ends that Hawke often captures something even more profound: the communion between singer and song, singer and songwriter, as the weight of what the performer has just channeled settles and shifts.

The titles alone are a map of the human heart: “Mama’s Hungry Eyes,” “If We Make it Through December,” “The Bottle Let Me Down,” “Going Where the Lonely Go,” to cite just a handful of his hundreds of compositions, 40 of them chart-toppers. Sad stories are the bread and butter of country music, but Haggard explored that terrain with an uncanny combination of directness and understatement — a Hemingwayesque “economy of words,” per the astute assessment of Bob Weir. Another interviewee, Taj Mahal, asserts that Haggard was a key figure in creating a new genre of blues.

At the wheel of his father’s gleaming black vintage Plymouth Barracuda, Hawke hits the road, California’s State Route 99, to be exact, in search of the music’s roots and the man’s. That north-south artery through the scrubland and ranches of the Central Valley, Hawke notes, connects key places in Haggard’s life. It was a primary thoroughfare during the ’30s, when the Dust Bowl drew hundreds of thousands of Oklahomans, including Haggard’s parents, to the Golden State. At stops along the way, and sometimes in the recording studio, Hawke reads aloud from Haggard’s two autobiographies, Sing Me Back Home and My House of Memories. No less than Haggard’s lyrics, his prose packs a wallop.

Working again with the deft editor Barry Poltermann, who cut Hawkes’ Newman-Woodward series, Hawke delves into archival riches that include docs made during Haggard’s lifetime. Dayton Duncan’s interview of the musician for Ken Burns’ 2019 PBS series Country Music is an especially strong element. The close-ups of Haggard’s handsome, weathered face not long before his death are powerful on any terms, but especially in the context of the life story, eloquently distilled here.

It begins with the primal wound of his beloved father’s death when Haggard was just 9, through the years of truancy and train-hopping and crime, multiple escapes from local jails and nearly three years in a maximum-security prison. Within nine years of his release from San Quentin, Haggard was not just a leading figure in the realm of the Bakersfield sound, but he was the top country musician in the United States, with three No. 1 singles in 1969 and a growing collection of industry awards.

The emotion builds in the doc’s second half with the high-flying Haggard’s ambivalence about success, guilt over his wealth and shame over his past, not to mention his profligate ways, bankruptcy and five marriages, some much briefer than others. The story of his relationship with Bonnie Owens, his second wife, creative partner and harmony singer nonpareil, is a love story for the ages.

Hawke conducts new interviews too, including with some of Haggard’s children and one of his wives. Among those musicians offering commentary but not performing are Haggard’s dear compadre Willie Nelson and the indispensable Dolly Parton, the object of Haggard’s unrequited love when they toured together.

Highway 99 is also a thoughtful counterpoint to the prevailing us-vs.-them oversimplifications that pigeonholed Haggard as a right-winger on the basis of his 1969 hit “Okie From Muskogee” (not unlike the way his contemporary Bob Dylan was saddled with being the voice of the counterculture). “Okie” was generally embraced by conservatives and reviled by hippies, the gray area between its earnest complaint and its cheeky humor rarely recognized. But Haggard, like most people who don’t adhere to strict party lines, had the guts to rethink his stances and reverse course. His comfort with contradictions and complications, Rosanne Cash points out, wasn’t readily accepted at the time, and certainly isn’t today.

Which brings this kinetic, vibrant documentary back to the music that Hawke, rapt and searching, holds forth as the key to understanding the man and the artist. It’s personal for him — it was from his father that he learned to love country music. Not every documentarian’s first-person contributions enrich the proceedings, but here the quest hits all the right notes, and the gathering of voices feels like a place to begin rather than a summing-up.