If not for a dinner in 1973, Henry Louis Gates, Jr., might today be summering on the Vineyard as a top doctor or prominent attorney, instead of a leading scholar of African American history and culture who’s also an award-winning documentary broadcaster.

At Yale University in the early 1970s, “I was pre-med and pre-law, like everybody else,” recalled the Harvard professor, author and host of the PBS documentary series Finding Your Roots.

A graduate fellowship at Clare College, University of Cambridge, England, landed Mr. Gates in the company of British-Ghanaian author Kwame Anthony Appiah (currently writing The Ethicist in the New York Times Magazine) and exiled Nigerian playwright-poet Wole Soyinka, who would become the first Nobel laureate from Africa in 1986.

“Both of them, in various ways, told me I had been put on earth to be a person of letters and I had to abandon that loony idea of being a lawyer or a doctor — at the same dinner on the same night,” Mr. Gates said.

He listened and he learned. “I studied African literature with Wole Soyinka, one to one. How lucky could you get?”

This June, Mr. Gates returned to Clare College. “I was deeply honored that they had decided to hang my portrait in the graduate common room at Clare. I just can’t tell you how deeply moved I am by the decision,” he said, before pausing a moment — a rare lull in conversation for a man whose mind, and person, are generally on the move. A canceled flight from Logan Airport to Chicago had opened up just enough time for him to speak with the Gazette by phone from his Cambridge home, where he was setting the table for dinner. Usually he would be on the Vineyard at this time, where he and his family have vacationed since 1981, but filming called.

“I think of myself as a professor and teacher first and a writer and filmmaker second,” Mr. Gates said. “I love them equally and I’m privileged to do them.”

The recently-completed season six of Finding Your Roots will air on PBS this fall, and season seven begins production “immediately,” Mr. Gates said. Tapping into the 21st-century fascination with genealogical testing, the series traces the family histories of influential Americans.

“I’ve been fortunate enough to be able to embark on a second career to express my thoughts through another medium,” he said. “I thank God every day that I’m able to make films.”

Mr. Gates screened clips from his upcoming series, Reconstruction: America After the Civil War, for an audience at the Beach Plum Inn following a recent fund-raising dinner for the Martha’s Vineyard Film Festival in Chilmark. His accompanying book, Stony the Road: Reconstruction, White Supremacy, and the Rise of Jim Crow, was published in April. He will discuss the book at the Martha’s Vineyard Book Festival on August 2 to 4.

“It’s all of a piece,” he said. “If I make a film, I’ll write a book and that informs what I teach.”

Reconstruction — the dozen or so years after the Civil War when black emancipation was extended to voting, political office, employment and other citizens’ rights — was quickly followed by the Redemption period, a white backlash that unleashed the oppressive Jim Crow regime, lynchings and further horrors on black Americans.

Redemption, Mr. Gates writes, reached its “zenith in horror — the highest point of the lowest low — with the screening by President Woodrow Wilson at the White House in 1915 of D.W. Griffith’s The Birth of a Nation.”

“The film was an unapologetic, blistering attack on what the Redemptionist Griffith saw as the appalling tragedy that Reconstruction had been, represented through a dazzlingly effective marshaling of racial pornography in the emerging language of the motion picture.”

Mr. Gates does not let his words do all the work. Stony the Road contains page after page of full-color images from the period, many of them shocking to view: cartoon "pickaninnies" selling soap that whitens their skin, and “alligator bait” postcards depicting alligators attacking black men and threatening black children.

It only gets worse from there, including a lynching-themed valentine from the 1920s, complete with a real piece of twine tying the black caricature’s neck to the autumn branch above. More recent examples show a 1970s anti-busing flyer that warns: “Forced busing will result in a race of mulattoes.”

“I’ve been fascinated by and collecting these racist images for a long time,” said Mr. Gates, who sees his book not only as historical scholarship but as “an allegory for the rise of white supremacy today and the alt-right rollback of Barack Obama’s policies by Donald Trump.”

But these lurid postcards, caricatures, movie stills and book covers — early versions of today’s racist memes on social media — are not the book’s only illustrations. Its section on The New Negro, a race-reframing movement exemplified by the Harlem Renaissance, includes dignified photographs curated by W.E.B. DuBois and sketches of black educators, artists and activists from the 1920s.

Mr. Gates’s next four-hour series and book project will focus on the black church. Its title comes from the Christian hymn Blessed Assurance, but Mr. Gates had to quickly run through the lyrics to make sure he got the words right.

“This Is My Story, This is My Song,” he said, returning to the phone.

After his travels, Mr. Gates will be back on the Island August 1, in plenty of time for his appearance at the book festival.

“I like book festivals in general, but on the Vineyard, the crowd for the screening and the book discussion at the Beach Plum Inn, some of the most intelligent and famous people in the world are sitting in your audience. You can’t have a more sophisticated audience than you can have on the Vineyard,” he said.

And, he added, “The Vineyard’s home for me.”