The new Grateful Dead documentary Long Strange Trip is essential viewing for Deadheads, but it's not a blissed-out concert movie, and its long, strange story is also surprisingly sad.

The film runs four hours – three of those covering Merry Prankster lore, grinning testimonials from fans like Al Franken, and a thorough airing of the band's history. Elements of tragedy begin to creep in as Long Strange Trip concludes its examination of the freewheeling ethos that founder/guitarist Jerry Garcia developed for the band and for its fans – an ethos the Grateful Dead honored even when it became apparent they'd created the proverbial monster (the Frankenstein story becomes a consistent and useful motif in the film).

The band refused to impose order on the subculture they created, and because they refused to control it, it began to control them. Among the casualties, the film suggests, was Garcia.

"Part of what I hoped to do within the film is to paint the Grateful Dead story with some finer shades of light and darkness. I worry that the Grateful Dead story has ignored some shades of darkness in order to stay aligned with the almost Pollyanna-ish, happy-go-lucky image that has stuck to them. That it's all good and nothing bad happens. Well, bad things happened," said director Amir Bar-Lev (who also directed The Tillman Story and Happy Valley).

The final hour chronicles Garcia's decline, tied to a grueling concert schedule, still doing 80 shows a year for fans he served in ways not always reciprocated – there is footage of Garcia pouring his soul into an apocalyptic dirge as drunks hoot and holler heedlessly in front of the stage.

"When Jerry Garcia refused to put his foot down regarding certain ways of coming to concerts or freeloading or behaving in antisocial ways, it allowed for people to come and make a mess of the ecosystem," the director said.

Other band members wanted to impose a loose set of rules, but Garcia refused to endorse them, saying they betrayed the band's antiauthoritarian principles.

Those principles, Bar-Lev said, keep the band still important and essential.

"I feel that in a lot of ways, they represented a threat, and still do. An alternative to mainstream, conformist, consumerist American fare. I think that at their heart, the Grateful Dead has just as much of a rebellious streak as punk and early hip-hop and other forms of music that have a well-deserved reputation for being formally inventive, and culturally relevant, and threatening to the status quo."

Bar-Lev devotes a great deal of time in Long Strange Trip to what Garcia called the "living, dynamic" nature of its music, improvised and meant to be experienced live, which put the band at odds with record labels, and with the profit model in general. Their guiding principle of being in the moment, Bar-Lev said, made them radical. And it's what makes them endlessly relevant.

"If you look at youth culture today, young people spend all of their time thinking about how they are being perceived, and this is intensified by social media. The Grateful Deal intentionally designed a place where you could be yourself and nobody was watching you and you were accepted. Teens today are constantly curating their lives, which is the opposite of what the Grateful Dead represented. They were committed to the present moment, and I feel like the culture yearns for that."

Bar-Lev was a teenager when he became a fan of the band, and he wishes his own children could absorb what was positive about the experience of being a Deadhead.

"I think I made this film more as a parent than as a fan. The phone is its own kind of drug epidemic. The Grateful Dead made some obvious mistakes regarding drug use and their enthrallment to escapism, but I worry way more for my kids and their relationship with social media and their phones than I would if they were at a Grateful Dead concert."

Bar-Lev also made Happy Valley, filmed after the Sandusky scandal at Pennsylvania State University, and he says the wildly disparate films have things in common.

"Grateful Dead concerts had something in common with PSU football games: They were both places where people come to enjoy being around other people. In our culture, that is getting harder and harder to do. Also, at a game or at a concert, you never know what's going to happen. It's not scripted. And that's part of the enjoyment."

Bar-Lev labored for four years to finish Long Strange Trip, coproduced by Martin Scorsese, and barely finished the movie in time for a premiere at Sundance, where Amazon bought it. After its one-day theatrical run on Thursday (locally at the Prince Theater and King of Prussia Stadium 16), and a week in New York and Los Angeles theaters, the movie will be available to Amazon Prime members.

Bar-Lev is amazed to be partnering with Amazon.

"After all that, it gets purchased by somebody who could push it out to the entire planet. The Deadhead inside me is extremely happy that some teenager in some far-flung place around the globe will be exposed to this inherently countercultural story. I feel like the counterculture on some level is getting the last laugh when Amazon puts this virus into the brain of some teen halfway around the globe."