"They've turned me into the psycho I've become," complains the unsteady voice in a taped phone conversation. "They" are the entertainment industry. The "psycho" is Larry "Wild Man" Fischer, a manic-depressive paranoid schizophrenic with a peculiarly haunting musical talent. This documentary by first-time filmmakers the Ubin Twinz (director Josh Rubin and producer Jeremy Lubin) tracks the life of this LA street freak who recorded an album with Frank Zappa in the 60s and once featured at number 48 in John Peel’s Festive Fifty with his catchy jingle "Go to Rhino Records". On screen he looks at first just like any other bum or crazy - the sort you see (and try to avoid) in any major city. In the experimental "outsider music" scene of LA, however, he was given a chance to be heard. But his albums were commercial failures, demanding an acute acquired taste. The talking heads assure us repeatedly that the man is a musical genius in what initially feels like a ruse in the mockumentary mould of Christopher Guest; but by the end we are inclined to agree.

The jangling dissonance and cracked-to-the-point-of-breaking timbre of Fischer's voice is remarkably penetrating. His songs began in the lonely bedroom of his childhood - troubled by the death of his father and an unloving mother; and they still sound that way performed by the man in his late fifties. What makes them so striking is their utter sincerity.

Rubin pieces together archive footage and interviews with a seamstress's flair. The story arc of the troubled genius/underground also-ran will be familiar from other recent documentaries about LA's subculture: Dogtown and Z-Boys (2001), DiG! (2004), and Mayor of the Sunset Strip (2005); but the Ubin Twinz sew their material together with fresh ingenuity and humour, contriving hand-puppets to illustrate an account of Frank Zappa's appearance on Dr. Demento's radio show.

Such proximity to a man with serious mental illness has the air of voyeurism and the very exploitation Fischer bemoans in the entertainment industry; yet Rubin answers this claim by pointing out he is exposing Fischer's music to a wider audience. In comparison with his TV appearances in the 60s - when he was laughed at by audience and presenters - this treatment is much more sympathetic. The film's coda has the personal affection of a Nick Broomfield documentary: the filmmakers have obviously befriended Fischer and gained as much of his trust as a paranoid schizophrenic is able to give, portraying his decline with pathos and sensitivity.

The most moving scene occurs when Fischer is reunited with his aunt Josephine after a number of months roaming the streets. Most of his immediate family have effectively disowned him after a series of violent attacks and repeated let-downs. He sings with tearful sincerity an apparently impromptu composition, a half-chewed bite of sandwich still in his mouth (we suspect his first proper food for days): "Sometimes it's sunrise. Sometimes it's sunset." In other cultures such poetic insight would be valued as the wisdom of a Brahmin or shaman. In the crude commercialism of LA, Fischer's behaviour will only be tolerated so far. He has a certain freakshow appeal; but his mind is lost (derailroaded) in some other reality on the other side of the tracks.