Time has a weird way of nullifying some truths of history. Over years, decades or centuries, narratives can be formed that feel so good they have to be true, despite the facts that suggest otherwise. Like the idea that Ben Franklin discovered electricity (read about William Gilbert) or that humans only use 10 percent of their brain (we don’t). Or that James Hetfield and Lars Ulrich of Metallica are the most important people in the history of metal music.

That last one is at least a little understandable. As the two most charismatic and vocal members of the most consequential metal band ever, Hetfield and Ulrich are going to reap glories they don’t deserve. But watch the new documentary “Murder in the Front Row” and you quickly realize they’re probably not even the most important people in their own band. That honor could belong to longtime guitarist Kirk Hammett, who just might be the most influential figure in the history of thrash music.

The film, scheduled to have its world premiere at 1 p.m. Saturday, April 20, at the AMC Kabuki 8 in San Francisco, is an early valentine to the Bay Area metal scene of the ’80s, when the bands bubbling up from the East Bay weren’t just providing a soundtrack for a collection of active misfits; they were building a blueprint for an entire rock subgenre. Groups like Exodus, Testament and Metallica, who made the move north from Los Angeles, were exploring their fret boards and kick drum pedals for sounds and rhythms that echoed the angst and frustrations of a bizarre era of nuclear armament and crumbling economies. But look closely; the guy behind much of it was Hammett.

“At one time, Kirk Hammett was the central driving force of Bay Area thrash music,” said the film’s director, Adam Dubin. “It’s funny because as time has gone on Lars and James (get most of the attention), but you could make the case Kirk designed much of what would become this music.”

Back in the Reagan years, he was just another kid from El Sobrante complaining about how little there was to do in the Contra Costa County suburb. So Hammett got into music, learned to play guitar, heard Michael Schenker shred on some UFO records and began searching for some fellow misfit rock fans to close ranks in a band he dubbed Exodus. In the process he introduced a guitar style that soon became a hallmark of the scene: fast, exacting and piercing.

“He taught me how to play guitar,” Exodus guitarist Gary Holt says in “Murder.” “We do it exactly the same because he showed me how to hold a pick.”

There are no strong statements of credit in the documentary, but sit through half of this 90-minute film and it’s clear Hammett was the catalyst to a scene that redefined heavy music for the world by melding the urgency and irreverence of punk with the technical precision and sonic bombast of metal. He found vocalists. He helped coordinate shows. He wrote music. He connected musicians. And when Metallica needed a new guitarist, he was the band’s first call.

“I don’t want to put too much credit on him because I don’t think he would appreciate it,” said Dubin, who has known Metallica since he documented the making of the band’s “Black” album in the early ’90s. “And it would be a little misguided because he was one of many.”

Ultimately, that’s what this documentary champions, the range of people who bonded through the blood — literal and figurative — spilled during the concerts, parties and experiences that characterized the Bay Area thrash scene.

Dubin, who got his start shooting videos for the Beastie Boys, interviews a wide range of musicians, fans and club owners who came together in the churning crowds of these early concerts. Their shared stories, of shows at Ruthie’s Inn or parties at band members’ El Cerrito homes or the hours spent riding BART just to get to a record store, give image to a scene that has little video evidence.

“Your only invitation to entry in this scene was that you had to love the music,” Dubin said during a recent phone interview. “They didn’t care if you were a boy or a girl, black or white, gay or straight. It didn’t matter, I think they all felt like outcasts.”

The film starts with Hammett and Exodus but quickly shifts to the tape-trading culture of the Bay Area that connected its nascent sounds to the world’s metal scene. Guys like Brian Lew, who co-wrote the book “Murder in the Front Row,” formed that network of musical connections outside the Bay Area, but also girded the relationships in their own city by documenting these bands in fanzines or sending live recordings to fans in Europe.

One of the local heroes was Cliff Burton, the late wunderkind bassist who convinced Metallica to move to the Bay Area when the band recruited him to their fold. Dubin talked to Burton’s girlfriend and father, both of whom characterize Burton and his bandmates as what they were at the time when Burton died in a tour bus accident — young men who had never experienced that type of loss.

Burton’s void is filled with footage of Metallica’s 1985 appearance at Oakland’s Day on the Green festival. It wasn’t just an excuse to display Burton’s virtuosic abilities; that concert was a culmination of everything the Bay Area thrash scene had been building toward: a local thrash act playing in front of 60,000 people.

“It can’t be overstated what a huge moment that was, for not just Metallica, but for the whole Bay Area scene,” Lew says in the documentary. “It was like all of us were playing Day on the Green. … Metallica came out and they just took over.”

That could also be a statement about the band’s trajectory after the festival, as it evolved into one of the most popular bands in the world by playing a sound that originated in the Bay Area.

Fittingly, the last substantial words in the film come from Hammett.

“It was a perfect storm of all the people that we needed to form a long-lasting musical scene that went on to do great things. And the amazing thing about it is we were all young, innocent and didn’t know what … we were doing or where it all was going. It just happened. It was like, from heaven,” Hammett says, his voice starting to break from emotion. “It was one of the greatest times in my life.”

“Murder in the Front Row”: Documentary. Directed by Adam Dubin. 1 p.m. Saturday, April 20. AMC Kabuki 8, 1881 Post St., S.F. 415-346-3243. www.amctheatres.com

Metal Allegiance: Band of Bay Area metal musicians will play a concert following the documentary premiere. 8 p.m. Saturday, April 20. $26.50. The Fillmore, 1805 Geary Blvd., S.F. www.fillmore.com