You had to have been there the first time “You’re Gonna Miss Me” blared out on AM radios across the country. A few measures of mercilessly whanged guitar chords, clanking cymbals, and throbbing bass, followed by an unearthly burbling sound and then the shock of some spine-tingling screeches (like a cat with its paws stuck in a light socket) — all of this boggled millions of minds in the summer of 1966. Long before rock nerds would credit them as a precursor of punk and an influence on bands as diverse as the Butthole Surfers and the White Stripes, Roky Erickson and the 13th Floor Elevators rode a wave of feedback to the top of the charts, a megawatt inspiration to garage bands across the country.

At the time, “You’re Gonna Miss Me” was literally electrifying; the Elevators pioneered the fusion of blues, country, R&B, and jug band music into “acid rock.” The band’s prophetically titled first album, “The Psychedelic Sounds of the 13th Floor Elevators” introduced the term “psychedelic” to popular music. A year before the Summer of Love and three years before Woodstock, the Elevators had turned on and tuned in.

But then they dropped out. With the benefit of hindsight, “You’re Gonna Miss Me” is a cautionary tale about the perils of a one-hit wonder. The Elevators thudded to a halt in 1968, and Mr. Erickson dipped in and out of the music scene over the next two decades — and then vanished.

His decline began in 1969, when — after hundreds of LSD trips — he was arrested in Austin, Texas, for possession of one marijuana cigarette. Incredibly, he was advised to plead insanity to the charge and was incarcerated in a mental hospital, where for three long years he endured drug and electroshock therapy.

The toll those years took can be seen in a new documentary, “You’re Gonna Miss Me,” directed and co-produced by Keven McAlester, which opens Friday at Cinema Village. Mr. Erickson gradually slipped into a bizarre alternate universe of paranoia and dementia. His first marriage fell apart, then he married and divorced again. His second wife says in the movie that he betrayed her as if with another woman when he resumed taking drugs.

Still, he kept recording — two of his subsequent songs, “Two Headed Dog” and its flip side, “Starry Eyes,” were minor hits in 1975 — but darker and darker imagery crowded into his compositions. Around this time he paid a lawyer to draft a statement in which he declared himself an alien being. He wasn’t kidding.

Mr. Erickson kept writing and performing through the early 1980s, but after his last, reluctant public appearance in 1987, he disappeared for most of the next 15 years. In Mr. McAlester’s dispiriting film, we see a man who “walked with a zombie” (to quote a song Mr. Erickson wrote in 1980). He was walking all by himself.

Or rather stumbling around in a delusional fog inside a ramshackle apartment. Mr. McAlester’s unblinking yet strangely compassionate camera quietly settles on Mr. Erickson zonking out in his squalid quarters, music piped into his skull through headphones while cartoons play on old TV sets and tunes blare from a jumble of vintage electronic devices. Heaps of junk mail and stacks of boxed board games leave little room for the dull-eyed, matted-haired, potbellied Mr. Erickson to walk. But he has no particular place to go. He wears a visor and dark glasses indoors and plays with a Mr. Potato Head.

His mother, Evelyn Erickson, takes care of him when no one else will, but one of his brothers, Sumner Erickson, accuses her of denying him the drugs and treatment that could help him. As Mr. Erickson’s other three brothers and their estranged father drift in and out of the documentary, its focus shifts from the musician to the twisted dynamics of a family with some excruciating secrets that make the term “dysfunctional” seem sadly inadequate.

The film ends a year after a 1999 guardianship hearing to determine who is better able to care for Mr. Erickson, Evelyn or Sumner. It would be a dramatically satisfying third act if “You’re Gonna Miss Me” ended with Mr. Erickson back on his feet, his music career and psychological health restored. Unfortunately, the film leaves unanswered almost as many questions as it raises.

But during the seven years since the last scene was filmed, Mr. Erickson has written his own third act. In 2005, he appeared at the Austin City Limits Music Festival. Earlier this year, he performed for the first time in New York City. By all accounts, he has undergone an almost miraculous return to a state of artistic and personal recovery almost unimaginable from the film.

Last Saturday, as part of the annual River to River Festival, at Clinton Castle in Battery Park, Mr. Erickson played a dozen or so songs and a few encores to an enthusiastic and supportive audience. It would be condescending to review Mr. Erickson’s return to the stage as a successful case study in psychology rather than as a musical event. But it would also be an exaggeration to say that he played, or sang, as well as he did three or four decades ago.

Perhaps only Janis Joplin, a fellow Texan and occasional performer with the Elevators, could have matched his feral vocals back then. Today, at 59, Mr. Erickson can no longer torture his voice into the yelps and yowls that were his pre-institutional signature. He couldn’t reach the high notes in “You’re Gonna Miss Me,” opting to sing the wrenchingly plaintive repeated phrase, “I’m not coming home,” a full octave below his original rendition.

Still, Mr. Erickson rocked hard and heavy with the solid backup of his new band, the Explosives. As a guitarist, Mr. Erickson still has real muscle memory in his fingers, and his playing was clear and precise, though he performed two of his solos with his back to the audience. Throughout the concert he projected an endearing diffidence. His banter was mostly limited to repeating a heartfelt “thenk yew” in his Texas twang after each song. Otherwise, he stood around like an abashed boy, head down, shifting his weight from foot to foot, arms hanging at his sides, his palms awkwardly turned backward.

But the zombie from the film had faded (along with the extra pounds) into the past, where it belonged.

We are so much the prisoner of VH1’s “Behind the Music” clichés — the glamorous decadence of sex, drugs, and rock ‘n’ roll — that we almost don’t recognize a real human tragedy, and a modest though genuine triumph, when they’re unreeled on film and displayed in person before us. Mr. Erickson’s performance was no nostalgia act; he was not just going through the motions in a comeback tour. Watching and hearing him play was like opening a time capsule that had been sealed decades ago.

We did miss you, Roky, and we’re glad you’re back.