Historical memory being what it is in this time of perpetual distraction, it’s likely the average consumer of pop culture is unaware of how deeply it has been influenced by one Julius Henry Marx. Even the mention of his stage name, “Groucho,” won’t, I fear, occasion much in the way of recognition among those under age 50.

Yet it’s difficult to imagine 20th- and, yes, 21st-century entertainment without him. Between the iconoclastic comedies he made with his brothers and his time as host of “You Bet Your Life,” Groucho Marx had a profound impact on the creative imagination. Ask the Beatles. Ask Larry David. Ask the recently departed Queen Elizabeth, who saw fit to quote Groucho on the celebration of her 80th birthday: “Anyone can get old — all you have to do is live long enough.”

Perhaps an aspect of immortality is that one’s efforts are so thoroughly absorbed into the body politic that they’re taken for granted.

“Groucho & Cavett,” the most recent addition to the PBS series “American Masters,” isn’t the best means by which to convert the benighted. Amongst Marx fanatics, a going theory is that the Brothers — that would be Groucho, Chico, Harpo, and, early on, Zeppo — were already past their prime when Hollywood took notice. The Marxes achieved their initial fame as stage performers in vaudeville and on Broadway, forums more conducive to wild flights of improvisation than film.

Time travel being what it is, no one will be booking a seat for the 1924 run of “I’ll Say She Is” anytime soon. The movies persist, though, with “Monkey Business,” “Horsefeathers,” and the supernal “Duck Soup” offering the most unadulterated access to the Brothers’ freewheeling absurdism. A hint of that flair is extant in “Groucho & Cavett” when we see a clip of the elderly Groucho purposefully stray off his stage marks to engage with the camera crew and members of the audience. The rules of TV talk shows were made to be flouted.

“Groucho & Cavett” is culled from the comedian’s appearances on the programs Dick Cavett hosted during the late 1960s and early ’70s. Mr. Cavett had been bumping around as a comedy writer since 1961, working as a staff writer on “The Tonight Show” and providing material for Jack Paar, Johnny Carson, and Groucho Marx, who served as guest host. Groucho took a shine to the young comic. It didn’t hurt that Mr. Cavett worshiped the ground upon which Groucho trampled.

The documentary features Mr. Cavett, still incisive at age 86, speaking to the trajectory of his career in comparison to that of Groucho’s: Mr. Cavett’s was on the ascendant, while Groucho’s was coming to a close. Marx suffered from ill health toward the end of his life — he died in 1977 — and his faculties suffered as a result. Still, Groucho was in fine fettle on the earlier episodes of “The Dick Cavett Show,” warbling “Lydia The Tattooed Lady,” talking smack about Richard Nixon and Lyndon Johnson, sparring with Truman Capote, and, as expected, trading in an abundance of quips.

The most moving segments of the documentary are when the comedian engages in rare moments of sincere self-reflection, from acknowledging the travails of aging (“I don’t belong in this world. I’m an incongruity.”) to the evident pride taken in having his letters acquired by the Library of Congress. At one point, Marx, who never completed elementary school, calls himself a “self-made man, a sad description of a man.” It’s a measure of his friendship with Mr. Cavett that someone who was among the most gifted and, in many ways, most guarded manipulators of the English language could admit to his vulnerabilities in a public forum.

“Groucho & Cavett” is a welcome addition to the Marxist canon. If the documentary isn’t quite as essential as the political fortunes of Freedonia, then it’s infinitely preferable to “Das Kapital.”