Joe Strummer is almost universally acknowledged as one of the good guys. As lead singer and guitarist for The Clash, he managed to preach without being preachy, creating in 1979's London Calling that rare album almost everyone can agree on. At that moment in music and history, The Clash became a cultural flashpoint for all the right reasons: sophisticated music; righteous sentiment.

Joe Strummer: The Future Is Unwritten is not a movie particularly interested in London Calling, which is as it should be -- we can geek out over known quantities on our own time. This biography revolves instead around Strummer's prodigious ego and attendant charisma; when asked how he wants to be credited on a TV interview, he replies, "I'd like you to write 'Punk Rock Warlord.'" And so he was: the film opens with footage of Strummer laying down the vocals for "White Riot," screaming them out without any instrumental backing. It's galvanizing stuff, filmed in 1976 by director Julien Temple, who promptly thereafter became the resident Sex Pistols chronicler and lost touch with the band. ("You had to choose, really," Temple told Alexis Petridis in an interview. "There was a lot of mutual slagging off.") The duo reconnected in 1997; after Strummer's death in 2002, Temple set about erecting this memorial.

The curious Mr. Temple is the closest thing the UK has to a national music historian. He's documented the Sex Pistols not once but twice (The Great Rock And Roll Swindle and its corrective, The Filth And The Fury), put out an anthology of performances from the Glastonbury music fest, and is working on "a kind of Rashomon" chronicle about ongoing turmoil between The Kinks' Ray and Dave Davies. He's also an art-damaged Jean Vigo disciple whose wish to make unconventional documentaries leads to occasional bad decisions. In Glastonbury, none of the bands performing were labeled -- you just had to be cool enough to know. Here, most of the interviews take place around campfires (a late obsession of Strummer's, who believed in their communal powers), and no one is labeled. The idea is to make everyone the same, basking in the warmth of Strummer's legacy and the fire. When Johnny Depp shows up in full Capt. Jack Sparrow regalia, or an unshaven John Cusack rambles about the punk ethos, the anonymous technique is upended: Thoughtful reminiscences are no substitute for star power, and Temple's form -- talking heads, great archival footage -- is far more conventional than he'd probably care to admit.

Still, The Future Is Unwritten is a nuanced and comprehensive biography; like most music docs, you have to care about the subject to care at all, but that's the only stumbling block. Filling in the gaps from schoolmates with audio interview clips of Strummer, Temple rambles through the childhood and the school years, envisioned with a nightmare anthology of clips from If..., Animal Farm and 1984. The prep-school nightmares fade away, but Orwell is a recurring motif, tying Strummer's increasingly paranoid and righteous behavior to the decay of Britain around him. Leap-frogging through industrial jobs in different towns when that was still an option, Strummer eventually settled into a squatter lifestyle, banging out rote tunes with the 101ers before being asked by punk svengali Bernie Rhodes to join The Clash. Some line-up shuffling and scrambling for fame later, The Clash became punk warlords and then a bloated stadium joke.

Temple's great contribution to the rise-fall-rebirth, Behind The Music arc is to make Strummer's downfall seem genuinely tragic rather than a simple case of drug-addled hubris: Here was a man whose ethical code was actively compromised by mainstream success and who broke up his band without financial incentive. (A revealing anecdote tells of Strummer's guilt and anger on finding out that "Rock the Casbah" was written on a bomb dropped in the Gulf War.) At the same time, he repudiated old acquaintances who didn't fit his newly-assumed punk ethos and slept with bandmates' girlfriends; principled generosity only went so far.

The expectedly turbulent Clash narrative gives way to a more complex if less dramatic story -- Strummer's transition from musically-blocked self-destructing prodigy to mature, married, at-peace elder statesman. In his last days, we see a bemused Strummer hustling on the Atlantic City boardwalk, trying to get clearly oblivious teens to come to a Mescaleros show. "Punk rock means exemplary conduct to your fellow human being," he announces in film-closing voice-over. By this point, it sounds like a hard-earned credo, and one worth believing.