Buddy Guy is among the modern-day blues guitarists in “Two Trains Runnin’,” a film that chronicles the search in the 1960s South for two pioneering bluesmen. (Avalon Films/Avalon Films and Freedom Road Productions)

In the summer of 1964, known as Freedom Summer, three groups of college-age American whites headed for Mississippi to register voters, sometimes guided by African Americans who understood the danger far better than the outsiders. On June 21, two of those Northern visitors — Andrew Goodman and Michael Schwerner of New York City — were murdered, along with black Mississipian James Chaney. In an uncanny coincidence, two pioneering yet nearly forgotten bluesmen — Son House and Skip James, whose whereabouts had long been unknown to music aficionados — were finally located on that same date.

Why, if there were three voting-rights excursions, is filmmaker Sam Pollard's evocative documentary about these events titled "Two Trains Runnin'"? Partly because that's the name of a classic folk-blues tune. But mostly it's because Pollard's documentary interweaves two separate but related phenomena: the civil rights movement and the rediscovery of country blues.

The abhorrent killings of Chaney, Goodman and Schwerner are a watershed that Pollard and writer Benjamin Hedin use to underscore the lesser-known events. The blues revival wasn't as cataclysmic as the civil rights movement, but like it, it transformed the relationship between blacks and whites in the United States (and beyond).

A scene from filmmaker Sam Pollard’s documentary. (Avalon Films/Avalon Films and Freedom Road Productions)

"Music was the forerunner," says civil rights chronicler Taylor Branch in the movie. "It was ahead of politics."

During the early-1960s folk boom, blues recordings from decades earlier were available mostly on old 78s, although a few had been reissued on LPs. While such musicians as Robert Johnson were famed for their demises, many of the others were still alive, but with whereabouts unknown.

In Boston, the MIT dropout, college-radio DJ and pioneering computer programmer Phil Spiro pondered clues that might lead to singer and guitarist Eddie James "Son" House. Spiro recruited two acquaintances and headed South.

In Berkeley, philosophy graduate student John Fahey (also a cult guitarist whose Takoma label was named for his Maryland home town) planned his second trip in search of musician Nehemiah Curtis "Skip" James. He hit the road with two friends.

The two parties of musical detectives nearly crossed paths in northern Mississippi. Fahey's group was arrested but released. Luckily for Spiro and his team — described as "three Jews in a VW with New York plates" — they learned their quarry no longer lived south of the Mason-Dixon Line, so they headed back the way they came.

"Two Trains Runnin' " presents this chapter of American cultural history with the usual mix of filmed artifacts, archival footage and contemporary interviews, supplemented by animated reenactments and songs by such performers as Chris Thomas King, Buddy Guy, Gary Clark Jr. and others.

The cartoon sequences are a little distracting, as is the flowery narration, read by rapper-actor Common. Also problematic is maintaining the appropriate tone, since Cheney, Goodman and Schwerner's murders outweigh even the most haunting old blues number.

But the movie's thesis is that the 1960s' political clashes and cultural revelations were essentially linked, and equally liberating. When those young, white blues fans found Son House and Skip James, they stumbled upon the past, but also the future.

Unrated. At Landmark's E Street Cinema. Contains images of racist brutality. 82 minutes.