Early in “Blackthorn,” I had to smile when the line “I can’t trust someone with no name” was uttered. Immediately, it brought to mind Clint Eastwood’s Man with No Name character from Sergio Leone’s “Dollars” trilogy — some of the best Westerns ever made. And “Blackthorn” is nothing if not a cinematic celebration of the once-vibrant genre, most notably 1969’s “Butch Cassidy and the Sundance Kid,” for which this serves as an unofficial sequel.

You may not recall — because Paul Newman or Robert Redford were replaced with the lower-wattage Tom Berenger and William Katt — that “Butch Cassidy” begat another chapter in 1979, a prequel since its true-life characters had died. 

How, then, does “Blackthorn” find a story to continue? Simple: Assume that Cassidy survived the standoff.

Screening Thursday through Sunday at the Oklahoma City Museum of Art, “Blackthorn” assumes that Cassidy (Sam Shepard, “Fair Game”) has been living alone and incognito under the name James Blackthorn in a Bolivian village. Sensing his time on this earth is limited, the self-described “ordinary old bandit” longs to return to the United States.

He withdraws all his money from the bank and sets out on his horse … and promptly loses both. This unfortunate situation forces him to align with a Spanish engineer-turned-thief (Eduardo Noriega, “Vantage Point”); heavy reluctance is trumped by threats of ever-increasing danger.

Although flashbacks show Cassidy in his younger days (Nikolaj Coster- Waldau, TV’s “Game of Thrones”) with the Sundance Kid (Padraic Delaney, TV’s “The Tudors”), the film directed by Mateo Gil (screenwriter of “The Sea Inside”) is its own beast, confident in its own tone. In other words, don’t expect Burt Bacharach singles and bicycle rides. Expect bursts of gunfire and kitchen-table surgeries.

Moviegoers also can look forward to big-screen beauty. Shot lyrically and taking advantage of Bolivian scenery, “Blackthorn” is a Western that delights in welcome views of wide-open vistas, in the comforting sounds of horses’ hoofs on cracked desert floors.

It also has what so many old Westerns lacked: plot and a fantastic lead performance. Whereas John Wayne commanded a presence, his acting often was deficient.

Not so with Shepard, who arguably has been given his meatiest role since breaking the sound barrier as Chuck Yeager in 1983’s “The Right Stuff.” His Cassidy doesn’t think of himself as an adventure hero who delights in past treasures, but a simple man who values friendship above all else, especially with the golden-years benefit of hindsight: “I’ve been my own man — nothin’ richer than that.”