The Grateful Dead have been such fixed dark stars in rock ’n’ roll’s cosmology that it’s surprising there has never really been an extended cinematic exploration of the band.

“Long Strange Trip,” ambitiously assembled and elegantly directed by Amir Bar-Lev, fills that void. The band’s main four surviving members — Bob Weir, Phil Lesh, Bill Kreutzmann and Mickey Hart — are all credited as executive producers and speak at length; Dennis McNally, the band’s publicist and biographer, whose similarly titled history was clearly consulted, is a presence as well. A bountiful trove of archival images and rare footage sketches their communal life offstage and the counterculture in which they played so formative a part. Crew members, family and fans share memories and lore.

The Dead, Mr. McNally contends, was “the greatest American band,” mixing rock, blues, folk, soul, R&B, gospel, country and jazz into an improvisational musical gumbo that was risky and uneven but that promised something different, possibly transcendent, every night. Even so, Mr. Bar-Lev tells a familiar industry tale. The road and alcohol and drug use take a lethal toll on the band: first Pigpen McKernan; then Brent Mydland; then finally its charismatic, shamanistic lead guitarist, Jerry Garcia, in 1995.

The film’s four hours (with an intermission) cover a lot of ground: Garcia’s teenage years in a bohemian Bay Area; the group’s roots as a jug band; its members’ service as house musicians for the Acid Trips of Ken Kesey and the Merry Pranksters. Despite driving its music label crazy, the Dead eventually rise to the top of the touring charts, playing for monster stadium crowds through the 1980s and into the ’90s.