The January 13, 1968 concert documented on the live album At Folsom Prison was not Johnny Cash's first performance at a prison, nor was it his finest. (That honor, arguably, belongs to 1969's At San Quentin.) But Folsom marked the first time one of Cash's prison concerts was recorded, and although nearly everyone involved thought the project contained no commercial potential, it became a milestone in Cash's career. The documentary Johnny Cash at Folsom Prison was originally bundled as the third disc on last year's CD/DVD re-release of At Folsom Prison, but now the film is making the rounds as a standalone feature—and proving, yet again, that the market for anything regarding Cash is inexhaustible.

The movie is not especially penetrating; it's more of a puff piece using archival photographs and current-day interviews. Animated videos of songs from the album—complete with sing-along lyrics—are particularly useless additions, but most frustratingly, the film leaves Cash's motives unexplored: What was his connection to the inmates? (It bears repeating that the Man in Black himself never did any hard time.) Why did he feel compelled to play for them? "There but for the grace of God," supposes one talking head, and the filmmakers leave it at that.

An interesting subplot develops in the story of Glen Sherley, an inmate at Folsom during Cash's 1968 concert. Cash performed Sherley's "Greystone Chapel" during the show, and later championed his release from prison. When Sherley was freed, Cash helped him get into the music business—but like so many inmates, Sherley couldn't cope with life outside of the cell walls, and shot himself in 1978. Interviews with Sherley's surviving family gain emotional purchase but, again, the movie offers no idea as to how Cash felt about any of this. Without any new or valuable insight, Johnny Cash at Folsom Prison has little option but to leave the viewer with a nagging sense that Cash used the novelty of performing for hardened criminals as little more than a way of inflating his own notoriety.