On October 4, 1986, the University of Alabama hosted Notre Dame in a game of football. Notre Dame had won the previous four contests, but this time Alabama was favored. It had a stifling defense and a swift senior linebacker named Cornelius Bennett. Ray Perkins, Alabama’s head coach, said of him, “I don’t think there’s a better player in America.”

Early in the game, with the score tied, Bennett blitzed Notre Dame’s quarterback, Steve Beuerlein. “I was like a speeding train, and Beuerlein just happened to be standing on the railroad track,” Bennett told me recently. Football is essentially a spectacle of car crashes. In 2004, researchers at the University of North Carolina, examining data gathered from helmet-mounted sensors, discovered that many football collisions compare in intensity to a vehicle smashing into a wall at twenty-five miles per hour.

Bennett, who weighed two hundred and thirty-five pounds, drove his shoulder into Beuerlein’s chest and heard what sounded like a balloon being punctured—“basically, the air going out of him.” Beuerlein landed on his back. He stood up, wobbly and dazed. “I saw mouths moving, but I heard no voices,” he later said. He had a concussion. After Bennett’s “vicious, high-speed direct slam,” as the Times put it, Alabama seized the momentum and won, 28–10.

Following college, Bennett was drafted into the National Football League. Between 1987 and 1995, he played for the Buffalo Bills, and appeared in four Super Bowls. During his pro career, he made more than a thousand tackles, playing through sprains, muscle tears, broken bones, and concussions. I asked him how many concussions he’d had. “In my medical file, there are probably six.” The real number? “I couldn’t even begin to tell you.” Fifteen? “More.” Twenty? “I played a long time,” he said. “Every week after a game, I got some sort of headache.”

In 1996, he signed a thirteen-million-dollar contract with the Atlanta Falcons. He received weekly injections of Toradol, an anti-inflammatory drug. “It was magic—it made me feel like I was twenty-four again,” Bennett said. He helped carry Atlanta to the Super Bowl—his fifth. (A more dubious distinction: his team lost in every one.) In 2000, at the age of thirty-five, Bennett retired and moved to Florida. He lived in a hotel in Miami’s Bal Harbour area, worked on his golf handicap, and vacationed with his wife and friends in Europe and in the Napa Valley.

Several of Bennett’s football peers were having a far tougher time. Darryl Talley, a former Bills teammate, suffered from severe depression. Mike Webster, a Hall of Fame center for the Pittsburgh Steelers, had become a homeless alcoholic; he died, of a heart attack, in 2002. Three years later, Terry Long, another former Steeler, committed suicide by drinking antifreeze. Andre Waters, a former Philadelphia Eagles safety, killed himself with a gunshot to the head.

A neuropathologist named Bennet Omalu autopsied Webster, Long, and Waters, and detected a pattern: each had a high concentration of an abnormal form of a protein, called tau, on his brain. Scientists associated tau buildup with Alzheimer’s, but that disease ravaged the elderly. This was clearly a different pathology, and in a 2005 paper Omalu called it chronic traumatic encephalopathy, or C.T.E., which he categorized as a degenerative disease caused by the “long-term neurologic consequences of repetitive concussive and subconcussive blows to the brain.”

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The N.F.L. tried to discredit Omalu’s findings. The league had set up a committee for traumatic-brain-injury research, led by a rheumatologist with a medical degree from the University of Guadalajara; the committee insisted that there was “no evidence of worsening injury or chronic cumulative effects of multiple mTBIs”—mild traumatic brain injuries—“in N.F.L. players.” When Bernard Goldberg, of HBO’s “Real Sports,” asked a committee member if multiple head injuries could cause “any long-term problem,” the member replied, “In N.F.L. players? No.” At a congressional hearing, in 2009, Linda Sánchez, a Democratic representative from California, compared the league’s “blanket denial” about C.T.E. to the defenses once mounted by Big Tobacco.

Bennett, outraged by the league’s stance, joined the board of the N.F.L. players’ union. In 2010, he was elected to head the Board of Former Players, and he participated in heated discussions among league representatives, team owners, and players. “What the hell was a rheumatologist doing talking about head injuries?” he asked himself. Current and former players, he told me, harbored a “lack of trust” toward the league. In 2011, players launched a class-action lawsuit against the N.F.L., alleging that it had “ignored and concealed” evidence about the “risks of permanent brain damage,” and had “deceived players” into thinking that serial concussions did not pose “life-altering risks.” Bennett told Bloomberg News, “If the lack of information and negligence continues, you aren’t going to have moms let their little boys play football.”

His own son, Kivon, had just turned eleven, and was starting to play tackle football. Bennett was flattered (“I’d dreamed of having a son that followed in my footsteps”), but also anxious (“You never want to get that call”). Parenting is about providing children with opportunities while protecting them from harm, and few recreational activities put those impulses in opposition the way football does. Yet Bennett never considered trying to stop Kivon from playing. “This country is built on giving you a chance to pursue your dreams,” he said.

Kivon was big for his age, like his father had been, and performed well on his youth team. Bennett shared safety tips with Kivon: how to protect his head when tackling by hitting his opponent with his shoulder instead of his helmet; how to improve his footwork. “I always tell him, ‘Positioning, positioning,’ ” Bennett said to me. “If he’s going full speed and he’s positioned, I feel as though that’s safe football.” Above all, he stressed to Kivon that he should let someone know if he thought he’d received a head injury.

Bennett wanted to give Kivon the best chance to excel. In 2015, when Kivon was a high-school junior, he transferred to St. Thomas Aquinas, a prestigious Catholic high school in Fort Lauderdale. Kivon, a strong student, enrolled in Advanced Placement classes. He had recently discovered “Macbeth,” he told me this fall. “I like the way the story lines didn’t add up at first but in the last few scenes it comes together,” he said. He has a Twitter account, and in his bio he posts his G.P.A.—currently 3.7.

But Kivon went to St. Thomas primarily to play football. The school has produced more pro players than any other high school in the country. By the time Kivon enrolled, the St. Thomas Aquinas Raiders had won eight state championships and two national titles. Moreover, the school had embarked on a potentially radical experiment. The head football coach, Roger Harriott, had been instituting changes to make the game safer. He limited practices to ninety minutes, and got the school to acquire a pair of motorized human-size robots, wrapped in foam, which players could tackle, saving their teammates from unnecessary hits. Harriott hoped to put St. Thomas at the vanguard of football safety while remaining champions.

“Football is just a vehicle to make these kids better young men,” Harriott said. One day this fall, he told his team, “Ultimately, it’s for you to become a champion in life—a champion husband, a champion father, community leader, colleague.”

The robots, covered in black foam, are known as Mobile Virtual Players, or M.V.P.s. Photograph by Thomas Prior for The New Yorker Photograph by Thomas Prior for The New Yorker

Such talk pleased Bennett. “My son is getting something from Roger that he’s going to take with him the rest of his life,” he said.