Tim Berners-Lee may have the smallest fame-to-impact ratio of anyone living. Strangers hardly ever recognize his face; on “Jeopardy!,” his name usually goes for at least sixteen hundred dollars. Berners-Lee invented the World Wide Web, in 1989, but people informed of this often respond with a joke: Wasn’t that Al Gore? Still, his creation keeps growing, absorbing our reality in the process. If you’re reading this online, Berners-Lee wrote the hypertext markup language (HTML) that your browser is interpreting. He’s the necessary condition behind everything from Amazon to Wikipedia, and if A.I. brings about what Sam Altman recently called “the gentle singularity”—or else buries us in slop—that, too, will be an outgrowth of his global collective consciousness.

Somehow, the man responsible for all of this is a mild-mannered British Unitarian who loves model trains and folk music, and recently celebrated his seventieth birthday with a picnic on a Welsh mountain. An emeritus professor at Oxford and M.I.T., he divides his time between the U.K., Canada, and Concord, Massachusetts, where he and his wife, Rosemary Leith, live in a stout greige house older than the Republic. On the summer morning when I visited, geese honked and cicadas whined. Leith, an investor and a nonprofit director who co-founded a dot-com-era women’s portal called Flametree, greeted me at the door. “We’re basically guardians of the house,” she said, showing me its antique features. I almost missed Berners-Lee in the converted-barn kitchen, standing, expectantly, in a blue plaid shirt. He shook my hand, then glanced at Leith. “Are you a canoer?” she asked. Minutes later, he and I were gliding across a pond behind the house.

Berners-Lee is bronzed and wiry, with sharp cheekbones and faraway blue eyes, the right one underscored by an X-shaped wrinkle. There’s a recalcitrant blond tuft at the back of his balding head; in quiet moments, I could picture Ralph Fiennes playing him in a movie—the internet’s careworn steward, ruminating on some techno-political conundrum. A twitchier figure emerged when he spoke. He muttered and trailed off, eyes darting, or froze midsentence, as though to buffer, before delivering a verbal torrent. It was the arrhythmia of a disciplined demeanor struggling with a restless mind. “Tim has always been difficult to understand,” a former colleague of his told me. “He speaks in hypertext.”

He visibly relaxed as we paddled onto the water. Berners-Lee swims daily when it’s warm, and sometimes invites members of the World Wide Web Consortium (W3C) to “pondithons,” or pond-based hackathons. “We have a joke that if you get any number of them on the island, then they form a quorum, and can make decisions,” he said, indicating a gazebo-size clump of foliage. He spoke of the web as though it were a small New England town and he one of the selectmen. Berners-Lee raised his two children in nearby Lexington, the cradle of the American Revolution, and rose early for the annual Patriots’ Day festivities. “We took them to the reënactment on the Battle Green,” he recalled, “and the midnight ride of Paul Revere.”

The Founding Fathers idolized Cincinnatus, who was appointed dictator to save the Roman Republic, then peacefully returned to his fields. Berners-Lee is admired in a similar spirit—not only for inventing the web but for refusing to patent it. Others wrung riches from the network; Berners-Lee assumed the mantle of moral authority, fighting to safeguard the web’s openness and promote equitable access. He’s been honored accordingly: a knighthood, in 2004; the million-dollar Turing Award, in 2016.

Now Sir Tim has written a memoir, “This Is for Everyone,” with the journalist Stephen Witt. It might have been a victory lap, but for the web’s dire situation—viral misinformation, addictive algorithms, the escalating disruptions of A.I. In such times, Berners-Lee can no longer be Cincinnatus. He has taken up the role of Paul Revere.

“They thought they were safe,” he said, as the boat startled a flock of geese. Platforms had lulled users into complacent dependency, then sealed off the exits, revealing themselves as extractive monopolies. Berners-Lee’s escape hatch is a project called the Solid Protocol, whose mission is to revolutionize the web by giving users control over their data. To accelerate its adoption, he launched a company, Inrupt, in 2017. “We can build a new world in which we get the functionality of things like Facebook and Instagram,” he told me. “And we don’t need to ask for permission.”

Berners-Lee knows that the obstacles are formidable. But he’s pulled off a miracle before. “Young people don’t understand what it took to make the web,” he said. “It took companies giving up their patent rights, it took individuals giving up their time and energy, it took bright people giving up their ideas for the sake of a common idea.” The dock slid into view just as he reached a crescendo. Smiling, he set down his paddle. “Shall I drop you here?”