Every experiment sets out to prove a hypothesis, and Tom Brady formulated an outrageous one last March, when he chose to flee the empire he helped build in New England to sign with the Tampa Bay Buccaneers.

Going from a franchise with whom he had won six titles to one that had won a total of six playoff games, Brady believed that, at age 43, he could master a new offense, acclimate to a new team and conquer a new conference, all while the coronavirus pandemic limited in-person activity.

The most precious of N.F.L. baubles is the Super Bowl ring, and each of Brady’s — seven, the latest secured on Sunday night — reinforces an indomitable truth: When he has something to prove, he is just about unbeatable.

Flicking away the Kansas City Chiefs’ dynastic aspirations and the quarterback who poses the most credible threat to someday matching him, Brady guided the Buccaneers to a 31-9 romp that recalibrated his own standard for greatness in front of a partisan crowd at their home field, Raymond James Stadium in Tampa, Fla.

This Super Bowl win was, for Brady, almost certainly his hardest, his sweetest and his strangest, too, captured at the end of the most improbable season in N.F.L. history. The final game is always an exhausting, exhilarating conclusion to the N.F.L. calendar, but never before had so many events surrounding the field of play threatened to pause the nation’s most popular sport.

After the pandemic gripped the country last spring, the decision to play the regular season on time, as scheduled, was met with pushback and confusion. The N.F.L. plowed ahead, establishing health protocols and reinforcing them as the virus upended schedules, postponed games and infected more than 700 players, coaches and staff members — as well as Brady’s parents, Tom Sr. and Galynn.

Civil unrest over racial injustice roiled the country, emboldening players, coaches and owners to protest systemic inequality, with some players gathering to challenge the commissioner, Roger Goodell, to proclaim that Black Lives Matter.

Brady left New England behind and did not comment this season on the red Make America Great Again hat seen in his locker there in 2015, but he did say his relationship with President Donald J. Trump became “uncomfortable.” As Trump’s term came to a close, with some of his supporters leading an armed attack on the U.S. Capitol, Patriots Coach Bill Belichick declined to accept a Presidential Medal of Freedom from him, citing his conversations with the team “about social justice, equality and human rights.”

For this untidy heap of a season even to reach Sunday’s capstone, it was as if the N.F.L. struck a cosmic bargain: In exchange for plowing through a full 256-game slate without creating a closed environment in which to play, it would be granted the most tantalizing quarterback matchup in the Super Bowl era, Brady versus Patrick Mahomes, the best of all time against the best of this time.

The last two Super Bowl-winning quarterbacks had never faced each other, and in some circles the game had been distilled in rather crude, and imprecise, terms as a referendum on each of their legacies — as if Brady’s would be tarnished with a defeat, or if, four seasons into a glorious career, Mahomes’s was somehow linked to the outcome.

In delivering Tampa Bay’s second to win his seventh — more than any N.F.L. franchise, more than John Elway and his boyhood idol, Joe Montana, combined, more than Michael Jordan in the N.B.A. — Brady joined Peyton Manning as the only starting quarterbacks to win Super Bowls with multiple franchises.

“We came together at the right time,” Brady, who went 21 of 29 for 201 passing yards and was selected as the Super Bowl’s most valuable player for the fifth time, said on the field afterward. “I think we knew this was going to happen now, didn’t we?”

Mahomes had won 16 of 17 starts this season, but he and his team collapsed amid a deluge of penalties, drops and pressure from the Buccaneers, who, exploiting the Chiefs’ diluted offensive line, reveled in it, inflicted it, even embraced it. At halftime, Mahomes had 67 yards, and Kansas City trailed by 21-6, its 15-point deficit tied for its largest during his three seasons as starter, according to Pro Football Reference.

“They beat us pretty good,” Mahomes said. “The worst that I think I’ve been beaten in a long time.”

The only other time Mahomes’s team had been behind by that many points across his three seasons as Kansas City’s starter occurred in October 2018, in a loss to Brady at New England. Brady spent two decades there, where he and Belichick were the immovable objects of the postseason, winning six championships as the most famous quarterback-coach tandem of this generation.

What Belichick must have been wondering Sunday night as New England wept, watching Brady throw each touchdown to a former Patriots teammate — two to Rob Gronkowski, who came out of retirement for the chance to play again with his old pal, and one, just before halftime, to Antonio Brown.

Brady’s time in New England will forever be a part of him, but now he wears a skull and crossbones on his helmet, can dress in shorts to practice in the winter and reports to a 68-year-old coach, Bruce Arians, who, coming out of retirement to coach the Buccaneers, represents the stylistic antithesis of Belichick. When asked recently about pursuing Brady during the off-season, Arians responded with a rhetorical question: “Do you sit and live in a closet trying to be safe, or are you going to have some fun?” He added, “I think I’d have been smoking something illegal to really imagine this.”

Brady’s arrival in Tampa reflected a certain harmonic convergence, a confluence of foresight, audacity and serendipity largely alien to the Buccaneers, who hadn’t won a playoff game since steamrollering the Raiders for their only title after the 2002 season. Their quest was nicknamed Operation Shoeless Joe Jackson, a wink to the prophesy from the movie “Field of Dreams”: “If you build it, he will come.” Brady valued how General Manager Jason Licht had assembled a team that solved problems around him instead of asking him to solve them himself. “They were an organization ready to win,” Gronkowski said of the Buccaneers. “The players here were ready to win.”

The Buccaneers loaded up on playmaking receivers, linebackers who excelled in coverage and aggressive defensive backs who matured as the season progressed. Before it even started, their cornerbacks coach, Kevin Ross, wrote on a board all the quarterbacks they would be facing — Matt Ryan, Drew Brees, Aaron Rodgers and Mahomes, who torched them in Week 12 for 462 yards and three touchdowns. But that defeat proved to be an inflection point for the Buccaneers, who had moonwalked through the first three months, going 7-5, shuffling forward at the same time as they drifted backward.

They closed the season by winning their last four, then defeated three consecutive division champions — and two of Brady’s elite quarterbacking peers, Brees and Rodgers — on the road to advance to their first Super Bowl in 18 seasons. That team, like this one, teemed with defensive talent and needed an outsider, Coach Jon Gruden, to synthesize it into a champion. Brady conferred the Buccaneers with hope and credibility and possibility.

“Brady, there’s nothing that he can’t do ,” said outside linebacker Shaquil Barrett, who had four tackles and a sack. “New team, new scheme, first year, going to the Super Bowl and winning it all. He’s the GOAT for sure.”

In a measure of Brady’s sustained excellence, consider that he has now quarterbacked not only the most recent team to win consecutive Super Bowls, with New England after the 2003 and 2004 seasons, but also the last two to ruin repeat bids. If a classic defensive play foiled Seattle after the 2014 season, then it was a comprehensive effort that smothered the Chiefs.

Heading into Sunday, Mahomes had finished 14 drives this postseason, excluding kneel-downs, and only once did the defense prevent Kansas City from scoring a touchdown or attempting a field goal. Tampa Bay’s defensive coordinator, Todd Bowles, devised a masterly plan to neutralize Mahomes, erasing the sideline and deep areas while deploying a four-man rush.

The Buccaneers forced two turnovers, held the Chiefs without a play longer than 14 yards until the third quarter and prevented the team from scoring even a touchdown. The last time the Chiefs didn’t breach the end zone, in Week 11 in 2017, Mahomes was backing up Alex Smith.

“I think he got a little tired of hearing how unstoppable they were,” Arians said of Bowles.

Brady ousted three Super Bowl-winning quarterbacks during this championship run, but unlike Brees and Rodgers, Mahomes, 25, has plenty of seasons remaining to flaunt his exceptional talent. The league is now Mahomes’s, and with the Chiefs’ inventive coaching staff and nucleus of bountiful young talent, his time will very likely come again.But this championship belongs to Brady, who, 10 Super Bowls down, has already started plotting his off-season objective: He wants to get faster.

As he smiled the other day, Brady said he wanted to catch up with the younger generation of quarterbacks. Really, though, it is they who are trying to catch up with him, always and forever.